Running a vim script on the codebase you can just do this to each file.
%s/string_to_replace/replace_string/gc
Then you just click type 'y' or 'n' to replace files. The time is minimized to a couple key presses.
Sounds like you are in for a fun time. I love those kinds of programming problems, that are outside of a normal programmers comfort zone. Definitely time to learn meta-programming tricks.