In my experience a far more common and much harder issue is dealing with dependencies.
A large project probably has lots of them. If you are lucky there is a 64 bit version of it. Very often though there isn't so you have to find something equivalent and rewrite all interactions with it.
And there might have been very good reasons for choosing those specific dependencies.
That can take ages upon ages and can be quite demotivating. You might have issue even finding out if you have any problems in your own application after you've spent months on replacing dependencies.
Of course basic examples between 32 vs 64 are easy to find. I guess I should have clarified as the OP sounded like he knew the topic well.
I’ve never dug hard into C++ gaming architectures and the patterns they use.