If the pistons are in good condition, there is no real reason to replace them. Unless... if the rings have damaged the bore, you will definitely need pistons, because you will require a re-bore. But you won't know that until you pull it all apart.
You should be able to buy just the rings, but if you buy pistons, you will get a set of rings.
While you have it all apart, it's wise to also replace, at least, the big-end bearings, and maybe the crank bearings.