I went through that a few years ago and it actually went pretty smoothly. There were a few UINT_PTR or DWORD_PTR changes I had to get used to and a couple of string glitches (we mostly used the _T() macro for strings and already used the _t variants of string functions in the original code, so that helped).

The biggest problems were DAO (database) and a few COM controls that were not available in x64.