There were always tools to diff two codebases and make a intermediate file that ...

There were always tools to diff two codebases and make a intermediate file that had all the changes. We did it back in the BBS days. You could always take the diff file and type the changes in manually. That's how "mods" worked. It would be something like this

  SomeFile.pas
  14: - If Variable = 1
  14: + If Variable = 2