PCB - Printed Circuit Board. All games are PCBs.
AES conversion back to MVS... why would you want to do that? I admit my AES knowledge is limited, but afaik an AES conversion is worth more than the original MVS cart (people convert MVS titles to AES because the real AES version is very rare or non-existant), so just sell the conversion and buy the MVS version. Anyway, to answer the question, I would imagine you could reverse a conversion if you knew what you were doing and had a suitable pcb to put the chips back onto.