Depends on your code. For well-written code that conforms to standards, switching the compiler may be completely pain-free. For most code, you'll probably get a few warnings (since Clang's default warning set is different than GCC's). In rare occasions you'll hit an actual behavioral difference between GCC and Clang, but you'll typically only find those where you're already writing non-standards-compliant code.

Also, if you do get some warnings when you switch to Clang, I would recommend fixing them rather than trying to squelch the warnings. Clang's warning set is pretty well-thought-out and things it flags as warnings are typically indicative of bad code (or coding practices).

You'll probably get some minor warnings here and there due the differences in the Parser. But in my experience, it's been relatively painless.