thats why you should always know what your programing for...me, i get to decide what version we'll be using for apps so its all good
it shouldnt require a full rewrite though, could probably just modify some parts. any SQL statements should be the same (unless they added support for new commands in 4.1 from 4.0. its probably most likely the connection method. is this for a business site or your own personal site?
-Darth Simon