FWIW, every upper I buy has a complete bolt, carrier, charging handle, etc., from the same manufacturer, and is bought at the same time. I make a point of specifying this, and much prefer it to be so.
Granted it is more expensive, but it lessens the possibility of different manufacturers working to different specs. Shouldn't happen, but it sometimes does.
In the event of a problem, it also simplifies any possible warranty work. Just ship off the whole assembly (after all, it's all theirs!) and let them figure out where they screwed up. Haven't had that happen so far.
In theory AR bolts should be interchangeable. Mebbe so. In that case, I have a lifetime supply of bolts in case TSHTF
If you sell off an upper, makes sense to sell its matching (and perfectly functioning) bolt as well. A selling point IMHO. After all, the parts have worn-in together, and work fine. If it's a high mileage unit, than tell the buyer so, and adjust the price accordingly.
Never had a problem with approach, and changeover is a snap. No fumbling for parts, etc. Just put a rubber band from charging handle to front of "handle", for insurance, and change away.
YMMV.