We actually had a pretty good system for making changes like that.  First, we would change

the compiler to accept both the old and the new.   Then we would produce a warning

that on a particular date the old would no longer work.  Then we made the old an error

and printed a message about how to fix it.   Eventually, we just let it be a syntax error.

This process was applied many times on the way from typeless B to strongly typed C.