No, they are not the "same family". A 45nm reduction very probably means that all the input pin levels have changed (they could also not have changed, but I'd doubt it, considering we're on the embedded world).

I see two problems with your master plan:
A) Different processor families. Any single discrepance probably means your mod ends up as a frankenstein.
B) I disagree that soldering the processor is an "easy" task. For a start, try replacing the N900 processor with an identical processor. If you manage to unsolder it without breaking it, there's a job waiting for you at an assembly line, as said. Your über-human abilities will replace a quite expensive robot there.

Ah, don't break the memory either while you're at it. It's on top.