I have an older Bushmaster M4 A2 that I bought probably 15 years ago .I was wondering if I was to buy a complete gas piston upper would I have to replace the buffer in the lower or any other parts to make it function or would the existing parts work with the new upper?  

Ask the people selling the upper? Most of the time it should be fine but there are no "standard" gas piston uppers so you are best checking with the maker

The only thing you would need to change is the BCG, that is if you don't buy it with the upper. The BCG for piston does not use a gas key but instead has a flat portion for the piston to push against.

I can take my Spikes upper with AA piston off my one lower, and swap with my D/I FDE rifle (one has a carbine buffer, other a rifle buffer) with no issue. Can even mount both on my .45 D/I lower, as it runs a standard carbine buffer.

 

Cannot really swap BGCs or specifically bolts, being my piston doesn't have gas rings on the bolt.

If you have a complete piston upper, with BCG, then you can put it on any lower.

What dose BCG stand for?

What dose BCG stand for?

Bolt Carrier Group