Preserve

Preserve the Behavior. Replace the Machinery.

If the valuable part of an old interface is its behavior, we don't need to rediscover every requirement before replacing it.

We can observe what the existing interface actually does.

Treat the old interface as a black box: give it inputs, capture its outputs, and use that behavior as the specification for a new implementation.

Preserve the behavior. Replace the machinery.

Regression testing at scale

The principle is simple.

Run the same inputs through the old and new implementations and compare the results.

When they differ, determine why, correct the new implementation and repeat.

This is regression testing — one of the oldest and most dependable techniques in software engineering.

The challenge is applying it systematically across thousands of real-world interfaces.

Some interfaces are straightforward transformations. Others interact with databases, stored procedures, files or external systems. In these cases, we need to capture or simulate enough of the surrounding environment to observe the complete behavior of the interface.

The objective remains the same:

Same inputs. Same observable behavior. Simpler implementation.

Run old and new side by side

Migration should not require a leap of faith.

The new implementation can run alongside the existing interface, processing the same real-world inputs without initially replacing it.

Differences can be detected, investigated and corrected while the existing production system remains authoritative.

Over time, this creates evidence that the replacement behaves correctly before anything critical is switched over.

The customer doesn't have to trust that a rewrite is correct.

We prove it against the system that already works.

AI changes the economics

Regression testing isn't new. What is changing is our ability to automate it.

AI can help understand existing implementations, generate replacement code, analyze differences and propose corrections.

But AI should not be the authority on whether the replacement is correct.

The regression system is.

AI proposes. Tests prove.

As these tools improve, more of the migration loop can be automated. What historically required substantial amounts of manual integration engineering can become a repeatable process.

A migration factory

The goal is therefore not another large rewrite project.

It is a migration process:

Observe → Reimplement → Compare → Correct → Prove → Replace

Each migrated interface improves the tools and processes used to migrate the next one.

That changes the economics.

Instead of asking customers to fund the manual rediscovery and reconstruction of decades of interface logic, we preserve the behavior they already paid to create and systematically move it onto a simpler foundation.

We don't need to reinvent 40 years of healthcare integration.

We need to preserve what works and replace what no longer needs to be there.