Other firearms different uppers

[my72ss 3]

Question ,  can you change out the upper on a 12.5 upper and put a 10.5  upper on it? Or  if you have a 10.5  upper can you replace that with a different 10.5 upper? 

[JackDaWack 2,895]

2 hours ago, my72ss said:

Question ,  can you change out the upper on a 12.5 upper and put a 10.5  upper on it? Or  if you have a 10.5  upper can you replace that with a different 10.5 upper? 

2 issues... 

A) will the new upper meet the OAL?

B) Can the upper not in use be used with any other combination of parts to construct an assault weapon?