[Arwill]

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.

[AstralStorm]

Unfortunately the swapchain and rendering is vastly different from DX9 to DX11, the latter being now more similar to Vulkan.

So maybe it could work to port the codebase to DX9 but no further really.