A developer announced excitedly that they had rewritten the project in the newest framework.
The senior developer asked, “What does it do now that it did not do before?”
“The same things.”
“And what does it not do that it did before?”
The developer paused for a long time.
What Are We Really Asking?
The rewrite is one of the most satisfying things a developer can do. The old code is known, its flaws catalogued, its compromises remembered. The new framework arrives clean, with better documentation and a community that has already solved the problems you spent last quarter struggling with. Merging that branch feels like progress.
The senior developer’s first question is the one we expect. What does it do now that it did not do before? Most of us prepare an answer for this, because we know it is coming. Faster builds. Better type safety. Fewer transitive dependencies. A clear upgrade path to whatever arrives next.
The second question is the one nobody rehearses.
Every codebase accumulates knowledge that never made it into the documentation. The odd conditional that handles a client whose data arrives malformed on the last day of the quarter. The retry logic someone added at two in the morning after a bad incident. The validation rule that looks redundant until you learn about the compliance audit from three years ago. This is institutional memory written in syntax, and a rewrite discards it by default. The new code does what the specification says. The old code did what the specification said, plus everything the team learned afterward.
The long pause matters. In that silence the developer stops looking at the code they wrote and starts looking at the code they replaced. That shift in attention is the whole teaching.
Before the next migration, try inverting the usual order. Catalogue what the current system already handles before you catalogue what you dislike about it. Read the commit history around the strange lines and find out why they exist. Ask who depends on behavior that nobody wrote down. If you can answer the second question honestly and the answer is nothing, proceed with confidence. If you cannot answer it at all, you have found the real work, and the migration is not it.