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.

(disclaimer: I work at Unity)

It is a button toggle problem, if the code does not do nasty things (like casting between a pointer and a 4-byte-integer -> all works until pointers are not 4 bytes). Or if a code depends on, for example, inline assembly (VisualC++ remove that in 64 bit). Or if the code generates code at runtime, for example JavaScript engines - for 64 bit they need to generate different code.

In Unity's case, the biggest holdup, of all things, was WebKit (used for asset store window & some other things). Turns out, there's no 64 bit webkit for Windows (or there wasn't a while ago). We had to replace Webkit with Gecko, which was quite involved task.