A viable solution for DX9 is using DXVK to emulate the old DX API under Vulkan, then add your own hooks into DXVK and transition to Vulkan in a more relaxed manner.

Now DXVK does not support DX7, but a quick search found dgVoodoo2, which does emulate DX7 under DX11. Maybe that or a similar library can be used as a stepping stone.

Regarding porting legacy apps to 64bit, the most problems i've seen were concerning old libraries (on Windows). That usually requires replacing old libraries with new a version, and fixing includes. I've seen only a handful of bugs arising purely from 32bit vs 64bit differences.