How "old" is your old BCG?
If you don't have 1000's or rounds on it, everything should be OK.
Swap the carrier all you want, but it's a good idea to keep a bolt to one upper. The bolt wears to the barrel extension.
In my experience anyway.
I agree... as rare as it may be for something to end up out of spec, I ALWAYS double-check with go/no-go gauges just to confirm that I'm GTG with a new barrel/bolt combo. An ounce of prevention is worth a pound of cure...Run your riles how you want, but I'm not going to give advice to someone else telling them "ya, it's hunky dory, go ahead."
For the $40-60 it costs to have one bolt per barrel or about the same for a set of GO, NO-GO gauges to check and make sure you are safe and just not haphazardly switching a bolt from barrel to barrel, that is what I'll do and recommend.
...The bolt wears to the barrel extension.
Maybe you have seen this, I haven't. I have one bolt that is on it's third barrel. And I usually change barrels at the 5,000 round, or so, point. Whenever 600 yard accuracy starts to go.
At this point, I'm not sure which BCG or upper it is in..The upper innards are all interchangeable no matter how old or how they have been shot.
In my experience anyway.