Sometimes it is, sometimes it isn't. The problem is knowing when it is or isn't. That needs building with all warnings enabled and vetting through them manually, a run with a static analysis tool searching for possible 64 bit bugs, or at worst, running extensive test suites to try to detect errors at runtime.

Some (older) software might also have some internal structures that are designed on the assumption that pointer size equals 32 bits.

This issue is made worse by the loose conventions in implicit integer type casts in C and C++. Warnings do help here, but there's going to be a huge amount of spurious warnings too.

So, at best, all you need to do is change compiler flags. At worst, you'll need a lot of manual labor in making sure that your applications build cleanly for 64 bit machines.