Hmmm...in PG if the param signature's the same it does what you want, that's the point of CREATE OR REPLACE. If you change the param signature then existing objects will call the old function (which won't be REPLACE'd because it's a different function due to the different param list). Maybe or maybe not what you want ... if you change the signature you should drop the old function if you're really meaning to replace the old with the new.
We don't have packages, which probably makes the problem a lot worse in Oracle ...
