That could work if the existing code is actually up-to-date, correct, and if you have knowledgeable users of the systems backed by this code. The part about 'code corruption' makes me thing this is not the case.

Worked at a big bank migrating some old code to new stack one time:

  - Original developers are gone.
  - Existing users don't understand the systems outside of their own narrow use-cases.
  - Code which was committed to repositories didn't match the actual running code, because people would go into production servers, manually edit the code there, in place, not document or commit the changes to repo. There were like 20 years of adhoc changes made in this way.

We had some business analysts which were supposed to talk to the users and gather requirements - they very quickly simply gave up and asked the developers to 'reverse engineer' everything.

Anyway, it was a hilarious shit-show, but kinda fun at the same time (if you're able to ignore incompetent leadership pressure - a skill I mastered during this time).