So if I transfer the contents of my current PC to one say 10 years older everything will work just fine and dandy?
because thats technically what your talking about doing with Data.
We don't know how primitive B4 was in comparison to Data. What we had was an on the fly analysis of their differences. We honestly don't know what hardware and software upgrades would be capable.
Not to mention its immoral as hell to overwrite a person just to save another one.
Immoral or not, it was already done in the film.