Subject: Pistons. Trash or treasure?

How do you know when a piston is no good? My pistons look ok, but on one
of them the top ring is half stuck in the grove. Can i just replace the
rings, or is this thing fried? If the one piston is bad do i need to
replace the WHOLE upper half of the engine on both sides? Seems everyone
just sells piston/cylinder sets. HELP....