I'm replacing an upper receiver from an AR-15 with a new one.  Is there anything at all to be aware of taking the BCG from the old upper and simply using it in the new upper?

Just wanted to be sure there's no best practice that you need to replace the BCG (wear mating, etc) when you replace the upper receiver.  I'm assuming not, but never hurts to double check with smart people.  Thanks.

Yes, I've put new Bcg in used upper and put used bcg in new uppers. No issues

Great. Thanks for the confirmation. ??

Yes, you can do this, because it's an AR-15.  

In other types of guns where the bolt, receiver, and barrel all need to be brought together correctly, like an M14 for example, you'd probably run into trouble with headspace if you just flippantly swap bolts.  On an AR, the headspace is set by the parts as they were made, versus by how the parts were assembled to make the gun.

I've done it many times with no issues.

No such thing as wear mating. Headspace isn't a concern because that's set with the barrel extension and not the bolt.

Feel free to use used bolts with any barrel.

Headspace on an AR is set with the barrel extension. A bolt will make absolutely no difference.

totally wrong information on the AR platform. If that were the base BCGs would be sold in different lengths ;)

Designed by the man himself to be end user serviceable. Meaning parts could be freely swapped between rifles in the field to make repairs on the fly.

Never had an issue with doing this on an AR.

I tend to by top quality gear and as long as things are built to spec they are interchangeable.

its fine

As others have said, headspace is set by the extension and has absolutely nothing to do with the carrier or bolt.

If anyone is unconvinced then mate a new complete bolt carrier group to a new barrel.  This is 100% unnecessary but might make you feel better.

ETA: I completely understand why someone would ask the questions, and respect them for doing their research.  There are MILLIONS of AR rifles in US civilian hands.  In fact, I read over the weekend the ATF believes there are 5-11M AR pistols currently.  That is just pistols!  Companies like PSA have taken "home building" to an entirely different level.  It has introduced a lot of people to ARs that woud have otherwise not been interested.  I would guess a lot of these folks know very little to nothing about gunsmithing.  This combined with the omni-present amount of USED AR parts floating around the internet would likely result in countless lawsuits and posts about problems resulting from mixing new/used and home brewing junk if there was an issue resulting from using old/new components.  However, it is rare to see any of this. For reference, I built my first AR15 from a worn out USGI parts kit on a SGW stop sign lower sitting at my grandmother's kitchen table in 1984.  Parts kits back then came in a sealed blue plastic bag filled with a mix of M16 and M16A1 parts combined with dubious quality semi-auto fire control components.  Barrels could be H&R, SAK, GM Hydra or Colt, etc.  Bolts were usually Colt for some reason. Carriers were unmarked and I suspect they were some 3rd party replacements. Guns built from these kits just ran and ran and ran. Back then you really only had a couple of options if you wanted an AR15.  You bought a factory Colt (which I did later that year) or you built one from these kits on an (Olympic) SGW lower (or cast EA ... not Eagle Arms ... lower a few years later). Heck, I had one of these ran over by a tractor and it didn't even loose zero.  Anyway, back then there was a "staple" vendor at every gun show.  Actually usually two or three.  These guys would have bins FILLED TO THE TOP with surplussed M16 parts for your home build.  This went on for years so I'm assuming people weren't KB'g their home built guns compromised of "who knows how much use it had before it was DRMO'd" parts thrown together in someone's garage.  I won't even get started on the craziness that happened when Colt introduced the "HBAR" and everyone had to upgrade their "now worthless pencil barrel" A1 to an A2 or even worse HBAR.  Suffice to say, people were cranking off their barrels and replacing them while not even considering changing the BCG or bolt.

Sorry for the wall of text.  I am old and rambling.

QFT