I'll start by saying I am by far not a builder of AR's.  My familiarity with them is from carry/cleaning m16's and m4's in the service.  So, I never did a lot of changing parts out.  (I let the armorer do my "thinking" for me )  Now, on to my question.  Is it a bad thing to use the same bolt/carrier in two different uppers?  I know that interchanging bolts is a no-no.  I am wondering if say, I had 2 uppers, and one complete bolt/carrier/ch could I swap them as needed, and not have any issues.  I am considering getting myself a "pre-ban" type upper, and I figure I'd throw in what I have now, which only has about 100 rounds fired,  and save $100 or so.  If that makes any kind of sense.

Without trying to oversimplify this.....

As long as your barrels are both in good condition and within spec, and your bolt is in good condition, and within spec, you can swap bolt/carrier assemblies between two upper all you want.

This comes up all the time.  The issue is, that a bolt wears, and a barrel wears, and typically, it is adviseable to have a bolt for every barrel.  However, it is not necessary, and as long as your barrels/bolts are not overly worn or out of spec, then you should never have any sort of problem with swapping.

Likey not, but some say you could have accelerated wear due to differing wear patterns of the two uppers.  If both uppers started out new, and your bolt/carrier started out new, likely you would not have a problem.

I have swapped bolts and bolt carrier assy's among rifles in troubleshooting.  Never a problem, nor did I have a fear of a problem.

STILL, it would be best if you bought a bolt carrier assembly for each upper.

I can see being on a budget, and using one bca for both for a while, until you safe your pennies for another.  Won't hurt a bit.

To clarify, I would probably end up putting this group into the new upper, and sell the old one minus the bolt/carrier, etc.  I wanted to be sure it's totally "safe" to do so, and it sounds like it'd be ok.
Thank you for the info!