Hmmm, I've never done anything like this but I'd have thought there's a few things to consider.....
If you're thinking of buying a neck with block inlays and dropping the old inlays in, will the inlays be the same shape and depth as the holes? It might take some sanding/filling to work, but I think sanding the inlays, while not damaging the fretboard, if they were too tall would be a tough job.
How bad is the neck? It might be easier to remove the whole fretboard and put that on a new neck?
Tbh, I have no idea though. The folks in the repair forum would be the most helpful 😃