component footprint as a new board design

I believe what you are trying to do is fit a different part package into an existing board design with a different package footprint.  If my understanding is correct: I have done this a couple of times:
Example we had a device in a 48QFN but wanted to prove in the repackage of the same die in a smaller package 28QFN
My solution was to create a castellated board.  The bottom side had 48QFN footprint with mirrored pin out. The top side had the new footprint matching the new device.  I need a few inner layers for routing and the thermal via structure of course had to be changed to accommodate smaller epad of the 28QFN.  I hop this helps.

https://www.eurocircuits.com/blog/castellations-on-a-pcb/