Question about replacing BCG's

[Apollo11]

I wanted to post this for a range buddy of mine. If you wanted to replace a LMT BCG with a BCM should you simply replace the carrier, or get the whole thing from BCM? The rest of the parts are LMT, and the only thing in question is a MIM gas key with some metal chipped off the face. My thoughts were that the entire piece needs to be replaced, but he feels otherwise.

Also, when replacing a bolt after the gun has 1000+ rounds through it; does the new bolt need to be fitted, or the chamber head spaced? Or can you just drop a new milspec bolt in? The question comes from seeing those that carry a spare bolt, my guess is that it drops right in, unless the barrel is about shot out.

Thanks in advanced for the help.