Hi, yep you can just unscrew the innards of a pgp and swap them out. It's a good idea to keep the empty body because the pgp threads will on occasion split open.
As far as adjusting goes, it's fairly simple once you get the hang of it. You may find it tricky at first. You have to physically turn the sprinkler to the right where you want the water to go. Then use the tool to adjust how far you want to it to turn to the left.
You can buy them online here.
www.sprinklerwarehouse.com/Hunter-PGP-ADJ-4-PGP-Adjustable-Arc-Roto-p/PGP-ADJ.htm