I'm considering keeping some spare parts on hand.  How interchangeable are the parts in the upper assembly (e.g., bolt carrier, bolt carrier key, bolt assembly, firing pin)?

Thanks in advance.

Pretty much everything is interchangeable, with the exception of bolts. Some guys do it, the military manuals warn against it, and I do not practice it. Unless you have the gauges for checking headspace.    
                                                                                                                                                                    New barrel=new bolt.

just get some head space guages and you should be set with everything including bolts