If you are looking to take your CH and BCG out of upper receiver A, put them in upper receiver B, and then sell upper receiver A and never see it again, then go for it. On a low-round-count gun, swapping internals like this isn't really going to hurt anything.
If you intend to keep upper receivers A and B, and swap the internals back and forth based on what you want to shoot that day, don't do this. A bolt and barrel extension develop complementary wear patterns and once they do, the wear slows down. By swapping back and forth, you never develop those complementary patterns, and the wear on your bolt doesn't slow down. You'll be constantly "breaking in" your bolt, wearing it down at an accelerated rate.
If you really want two uppers then buy the internals to go with the new one. That way you won't be swapping all the time, and you'll have a spare BCG if the other one breaks. You'll be better off in the long run.