How to Choose a Testing Strategy for Legacy App Migration: A Step-By-Step Guide

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The legacy software modernization market is rapidly growing and is expected to reach $27.3 billion in 2029, growing at a CAGR of 15.9%. One of the main reasons is that organizations invest in modern technologies and seek to transform outdated systems to optimize business performance.

At the same time, modernizing a legacy system is a challenging task. These systems are usually resistant to changes, as they have a complex business logic and are shaped by years (or even decades) of regulatory and operational constraints.

Good news: You don’t need to rewrite a system from scratch at once. Legacy application testing is one of the best ways to improve system quality, comply with regulations, and add new features to software. 

A Testing Strategy for Application Migration: What to Test First

A smart testing strategy for a legacy application starts with optimizing what already works, not preparing for a full rewrite. The goal is to protect critical behavior so the system can be safely changed, optimized, or modernized over time. Here is what you need to test first.

Core business workflows that directly impact your revenue

Test the end-to-end flows that the business relies on every day (orders, payments, reporting, approvals). If these flows fail, the business will suffer.

Revenue and compliance-critical features

Prioritize functionality tied to revenue, billing, data accuracy, and regulatory requirements. Any compliance and regulatory errors can lead to fines and reputational harm.

High-change areas

Focus on modules that are frequently modified. These are the most likely to break and the ones that benefit the most from test coverage.

Fragile components

Test parts of the system with a history of bugs, manual fixes, or production incidents. Stabilizing them reduces support and maintenance costs.

Features users often interact with

Cover the screens and actions that real users interact with most. Many legacy issues surface at the UI level, even when the backend remains unchanged.

Third-party integrations 

Validate interactions with third-party systems, APIs, and data imports. These dependencies often change outside of your control; that’s why it’s crucial to regularly check their performance.

E2E Tests Vs Integration Vs Contract Tests for Legacy Software

Now that you know what to test first, the next question is: which type of testing will deliver the best results? Let’s review each type of testing, its benefits, and pitfalls for your modernization testing strategy.

Testing type	When to use it	Level of complexity	Level of risk
E2E testing	To validate user workflows, especially for business-critical processes. Ideal when modernizing or releasing large changes.	High – tests the entire system, including UI, backend, and integrations.	High – slow, fragile, and can break due to mino,r unrelated changes
Integration testing	To verify that modules or components work together, especially after refactoring or when multiple internal systems interact.	Medium – focuses on component interactions rather than full workflows.	Medium – less fragile than E2E, but can miss user-facing issues
Contract testing	To verify interactions between services or APIs, especially for reused modules or external dependencies.	Low – tests only input/output contracts between components.	Low – stable and fast, but won’t detect bugs inside modules or across full workflows.

E2E tests for legacy applications

End-to-End (E2E) testing validates the full application flow from start to finish, simulating real user interactions. 

The goal is to ensure that all components work together as expected and the whole system operates without bugs.  Let’s review the main benefits of E2E testing for legacy apps.

• Covers critical business workflows

E2E tests focus on the flows that matter most to the business, ensuring that revenue-generating or compliance-critical processes work as expected.

• Detects integration issues

Legacy apps often rely on multiple internal modules and external systems. E2E testing validates that all parts work together, spotting problems that unit tests alone would miss.

• Tracks the quality of updates

When modernizing or refactoring legacy code, E2E tests provide confidence that updates do not break core functionality.

• Improves user experience

By simulating real user behavior, E2E testing highlights problems at the UI and workflow level, where legacy issues often appear first.

As with any type of testing, E2E tests have their pitfalls.

When to choose E2E testing for legacy software

End-to-end testing is most valuable for legacy applications when you need to make sure that critical business processes work from start to finish. 

• Use it after unit or integration tests are in place to validate overall system behavior
• Apply it when modernizing parts of the system incrementally, providing a safety net without rewriting the entire app
• Ideal for stable or rarely changing modules; features and flows that are often changed may require shorter, targeted tests first.
• Apply it when the focus is on real user interactions, especially through the UI.

Integration tests for legacy applications

The main goal of integration testing is to verify that different modules or components of a system work together correctly. Unlike unit tests, which test individual pieces of code, integration tests ensure that data flows, interfaces, and component dependencies behave as expected.

Integration tests focus on the connections between modules, verifying that data flows and component interfaces work correctly. They are faster, more stable, and ideal for checking interactions in parts of the system being modernized or refactored.

Here are the main benefits of integration tests for legacy software.

• Catch interface issues early

Legacy systems often have poorly documented modules. Integration tests reveal broken connections between components before they impact users.

• Support incremental modernization

When updating or refactoring parts of a legacy system, integration tests validate that new changes interact correctly with the existing system.

• Faster and more stable than E2E

Integration tests are more specific than full end-to-end tests, so they run faster and are less fragile while still verifying multi-component behavior.

When To Choose Integration Testing for Legacy Software

• Use when refactoring or modernizing parts of the system to ensure new and old components work together.

• Ideal for complex systems with multiple internal services or data flows, where E2E testing is too slow or fragile.

• Works best as a middle layer between unit tests and E2E tests, providing confidence that the system’s building blocks function correctly before testing full workflows.

Contract Tests for Legacy Applications

Contract testing verifies that services, modules, or APIs are compatible with other components of the system. It focuses on the interface rather than the internal logic or full workflows, making it a precise tool for validating dependencies in complex systems.

Benefits of contract testing include:

• Detecting integration breaks before runtime

Changes in a legacy module can silently break other services. Contract tests catch these mismatches early, without needing full end-to-end execution.

• Reducing risk of downstream failures

Legacy systems often have hidden dependencies. Contract testing ensures that updates in one module don’t disrupt other components.

• Enabling safer updates

Teams can modernize or refactor parts of the system with confidence that contracts with other modules remain intact.

When to use contract testing for legacy software

Use contract testing:

• When refactoring or modernizing modules that interact with other parts of the system.

• For services or APIs that are reused in multiple workflows.

• When E2E tests are too slow or fragile, you still need confidence that components communicate correctly.

• Ideal for incremental modernization: contract tests protect dependencies, while integration and E2E tests handle broader behavior.

How to choose the best testing approach for legacy software

Choosing the right testing approach depends on the system complexity and modernization goals. Legacy systems are often fragile, poorly documented, and difficult to change, so testing should focus on protecting what matters most while avoiding unnecessary overhead.

Identify features that impact your business first

Prioritize critical business workflows, revenue-generating features, and compliance-related processes. Determine which modules matter to your business most and focus your testing efforts there.

Match the testing type to the goal

For example, use contract testing when you need confidence that interfaces and APIs remain compatible, especially for modules reused across the system. 

Use integration testing to verify that modules work together, especially after refactoring or updating dependencies. 

And it’s better to use End-to-end (E2E) testing when you want to ensure full workflows function correctly, particularly for business-critical processes. 

Combine various testing approaches

Use contract tests to secure component interactions before modernization, apply integration tests to validate multi-module behavior, and run E2E tests selectively on the most critical workflows to optimize business performance.

Start small and scale

Begin with high-priority workflows and features, and gradually expand testing coverage as the system is modernized.

Conclusion

Before choosing the testing type for a legacy application, it’s important to understand which features are critical to the business, such as revenue-driving processes, compliance functions, or key user workflows. 

Also, it’s important to know the main goal of your testing: whether you’re maintaining the system, refactoring parts of it, or modernizing incrementally. 

Finally, consider available resources, including team skills and test infrastructure, since these factors influence whether you can support slower end-to-end tests or rely on faster contract and integration testing.

Don’t Know What Type of Testing to Start With?