It almost certainly would completely break thousands of functions that have those magic letters hard coded into them in fortran, cobol, C or whatever. You might even find that clever people back in the 70s used other ways of representing those letters so you can't even search for them.

I've worked on codebases like this -- I've seen it take a team of engineers years to add a single digit to an ID number in a multi-million LOC mixed language codebase that was written in the 60s.

It's not that it can't be done, it's that it costs millions of dollars and takes years and you're never sure it's actually done because nobody has ever written a single test for the system and if you tried that would take years as well.