Best Software Development Companies for Improving Existing Web Apps Without a Full Rebuild
Most slow or outdated web applications don’t need to be discarded. They need a partner who can modernize the existing code bit by bit, while the product continues to generate revenue.
A complete rebuild puts feature development on hold for several months and often leads to a delayed launch. Incremental modernization, on the other hand, involves reviewing the app, stabilizing it, and then upgrading it in small, implementable steps.
This is particularly important for healthcare applications, where modernizing healthcare IT and integrating AI must occur while ensuring that patient data continues to comply with HIPAA regulations. Below, you’ll find ten agencies specializing in custom software development that excel in this area, including those offering AI-driven software modernization services, as well as an overview of options and pricing.
How we chose these companies
- We sought out vendors who specialize in improving software already in use, evaluating them based on eight criteria.
- Expert knowledge of legacy and existing code. The ability to seamlessly take ownership of code not written by our team and to first identify the real bottlenecks.
- A structured audit prior to any changes. An analysis of the code and architecture that results in a prioritized roadmap, rather than a proposal to completely rewrite the code.
- Gradual modernization methods. Refactoring, “strangler-fig” migration, and phased updates implemented without downtime.
- Modernization using artificial intelligence and AI integration. Using AI to analyze legacy code and generate tests, as well as adding AI capabilities to existing applications.
- Readiness to work in the healthcare sector and compliance with regulatory requirements. Experience working with patient data, integration of electronic medical records (EMR), and ensuring HIPAA compliance, including signing a Business Associate Agreement (BAA).
- Quality and security systems. Automated testing, CI/CD, and monitoring to ensure the security of incremental changes.
- Proven track record. Examples of successful projects, years of market experience, and independent client testimonials.
- Flexible collaboration options and transparent pricing. Audits, fixed-scope projects, or dedicated teams with clear pricing structures.
The 10 best companies for improving existing web apps
1. IT Craft
Founded in 2001 as a one-person server administration shop, IT Craft now runs dedicated app modernization and project rescue practices. For buyers comparing custom software development agencies specializing in healthcare IT modernization and AI integration, its appeal is a record of upgrading live systems rather than replacing them.
The clearest example is Adorama, the photo and video retailer: the firm modernized and expanded its custom legacy e-commerce platform without pausing business operations, then stayed on for ongoing development, QA and maintenance. In healthcare, its teams finalized Aya Healthcare’s AI-powered clinic staff management platform, integrated it with existing EMR and staffing systems, and took over upgrades afterward. Its .NET and PHP practices cover legacy code review, stack upgrades, CI/CD rollout and performance and security fixes.
For a CTO inheriting an aging app, that combination addresses the main risk of modernization: downtime and lost functionality while the codebase changes underneath users.
IT Craft cites a 4.9/5 overall review rating, and one client credited the team with “full ownership and execution” of their product plans.
Best for: startups, mid-sized companies and enterprises, including health-tech teams, that need a revenue-critical web app stabilized, extended with AI features and modernized in stages.
2. Thoughtworks
Thoughtworks is a global technology consulting firm founded in Chicago in 1993, best known for shaping the industry’s approach to software development. Its chief scientist, Martin Fowler, popularized the “strangler-fig” pattern—a standard method for phasing out legacy systems—and the company publishes the popular “Technology Radar” overview.
Today, Thoughtworks combines this engineering legacy with AI/works—an agent-based platform for modernizing large systems—and partnerships with AWS, CAST, and Mechanical Orchard to modernize legacy systems and mainframes.
When it comes to existing web applications, the company’s teams typically start with a rapid assessment of the legacy system and then modernize or replace it in stages without disrupting business operations.
Best for: large enterprises with complex, business-critical platforms that need a structured, long-term modernization program.
3. Endava
Endava is an engineering company headquartered in London with 25 years of experience in digital transformation. “Core modernization” is one of its key areas of focus, delivered through the Dava.X initiative and AI-driven infrastructure.
Endava excels in highly regulated, high-transaction-volume sectors such as payment systems, banking, insurance, and telecommunications, where systems cannot be taken offline during upgrades.
Its teams work alongside clients’ engineers to modernize core systems in phases, adding cloud, data, and AI capabilities to what is already in operation.
Best for: fintech, payments and insurance companies that need to modernize transaction-heavy platforms at scale.
4. Keyhole Software
Keyhole Software is an American consulting firm whose staff consists exclusively of experienced consultants based in the U.S. who have, on average, more than 17 years of experience. The company specializes in transforming legacy Java and .NET applications, as well as monolithic applications, into modern, cloud-ready systems without disrupting business operations.
Keyhole has supported over 250 clients; it reports that 78% of last year’s projects were completed for repeat clients, and it received the Clutch award for the best application modernization company in Kansas in 2025. Recent case studies include a migration from .NET to microservices and React modernization for government agencies.
In the healthcare sector, Keyhole has modernized mission-critical systems for medical facilities, and Northwell Health notes that the Keyhole team helped launch a major product update.
The range of services varies from staffing augmentation to forming full project teams and consulting work, so clients can engage either a single senior architect or an entire modernization team.
Best for: US organizations with mission-critical Java or .NET web apps that want senior, onshore engineers.
5. Onveda
Onveda is a German software development company located near Düsseldorf, founded in 2010. Its software modernization service allows clients to replace outdated platforms without interrupting operations, and each project begins with a two-week software audit that assesses the platform’s current state.
Onveda often takes over the management of platforms that a previous vendor was unable to handle, and the same team continues to support the system after launch. A long-standing partnership with LANXESS—for which Onveda built a procurement system in 2014 and rebuilt it for a second global rollout in 2026—demonstrates how the company works with software that has a long lifecycle.
Since the system is based on widely used open-source technologies, another team can later take over the code, limiting vendor lock-in after the upgrade.
Best for: EU companies with B2B portals, catalogs or configurators connected to SAP or other ERP systems.
6. Grid Dynamics
Grid Dynamics is a company specializing in digital engineering for enterprises, operating in the fields of artificial intelligence, data, cloud technologies, and digital engagement. Modernizing legacy systems is one of its core services, and the company’s clients include major enterprises such as Google, Macy’s, Jabil, and Merck.
The company has a particularly strong presence in the retail and e-commerce sectors, where it modernizes search, personalization, and omnichannel platforms built on legacy commercial systems.
Its consulting teams help companies migrate legacy systems to the cloud and integrate artificial intelligence and data platforms into them, rather than starting from scratch.
Best for: large retail and e-commerce businesses modernizing high-traffic web platforms.
7. 10Pearls
10Pearls is a digital development company specializing in artificial intelligence, headquartered in the Washington, D.C., area, with a team of over 1,400 employees across four continents. Its modernization services include technology assessment, platform refactoring, user experience (UX) optimization, and enterprise application architecture, all supported by quality assurance and self-healing test automation.
The company has been listed on the Inc. 5000 for seven consecutive years and works with clients such as PayPal, Verizon, and Elevance Health.
Healthcare is one of its largest sectors, with clients including Elevance Health, Amwell, and Hologic. The healthcare worker support platform it developed has reduced administrative costs by more than 50%, and it has also added agent-based artificial intelligence to an existing healthcare support platform—a good example of integrating AI without a complete system overhaul.
Teams operate under “nearshore” and “offshore” models, providing clients with flexibility in terms of costs and time zones.
Best for: mid-market and enterprise companies, especially in healthcare and finance, that want refactoring and AI integration with extra delivery capacity.
8. Stride
Stride is a New York-based engineering and consulting firm that is now part of Accordion and uses artificial intelligence to modernize legacy software. It tracks code behavior during execution to understand what the legacy code actually does, maps dependencies, and automatically generates regression tests to verify functional compliance, with engineers overseeing every decision.
In one example, Stride modernized a flight management application for a global cargo airline and eliminated 504 out of 505 security vulnerabilities in six months.
Stride is one of the vendors on this list that focuses most heavily on the healthcare sector. The company uses artificial intelligence with human-in-the-loop controls for HIPAA-compliant use cases, and one of its healthcare clients saves $360,000 per year thanks to Stride-built agents that handle patient inquiries via SMS.
The company’s engineers also train internal teams in AI development practices so that the client’s own developers can continue to improve the application after the collaboration ends.
Best for: healthcare and financial companies modernizing legacy apps while upskilling their own teams.
9. Emergent Software
Emergent Software is a Microsoft Solutions Partner based in Minnesota that serves midsize organizations in the U.S. The company offers application modernization, as well as custom development, migration to Azure, DevOps, and long-term application support.
Since Emergent supports its solutions through managed services, the company is the obvious choice if you need a single partner to modernize a .NET web application, migrate it to Azure, and ensure its continued smooth operation.
Each project is led by senior engineers, and the company uses AI-powered development to accelerate implementation.
Best for: mid-market organizations on the Microsoft stack that want one partner for modernization, Azure and ongoing support.
10. Advena
Advena is a leading engineering and consulting firm specializing in cloud technologies, DevOps, security, and software. Its primary area of expertise is the infrastructure layer of existing applications: in one case study, the company migrated a monolithic payment application to Kubernetes without any downtime.
In another project, a self-service CI/CD platform was created, which reduced order fulfillment time from several days to less than an hour for over 40 services—a prime example of improving the delivery process without rewriting the product.
For healthcare applications, the main argument in Advena’s favor is its commitment to regulatory compliance: the company strengthened the cloud infrastructure and delivery pipelines for a medical technology company, enabling it to pass a SOC 2 Type II audit on the first attempt in just eight weeks.
The company’s engineers work side by side with the client’s team to build pipelines and infrastructure that the team can subsequently maintain on its own.
Best for: SaaS, fintech and healthtech teams whose app works but deploys slowly, scales poorly or needs to pass a security audit.
