Not sure anything I know of fully fits your question.. but it COULD be Jack Chalker's "Four Lords of the Diamond" series, although there are no rings to retrieve. The main character has his personality cloned (basically) 4 times, and stuck into 4 bodies, each of which travels to one of the four planets of the Warden Diamond. On each, the 'wardens' (basically a planet-specific adaptation of a kind of virus) modified each person in a way consistent with the specific planet.
They were very much NOT volunteers, though, and each one, waking up thinking he was the original, became quite upset to realize he was simply a clone, but eventually reconciled it by deciding to follow whatever life they could have. The 'original' monitored what each did/thought/experienced all thru the series. The planets were used as Prison planets, and each of the cover identities was someone who was consigned to there, so they were pretty fully set to blend in.
A second series (also by Chalker) that could be what you are thinking of is the 'Rings of the Masters' series. Although not the primary focus of it, there was a way to modify people down to the cellular level, that allowed them to be changed into any other life form, and a number of the characters did so, in their quest to retrieve the rings; it's a one way change, as multiple changes to the same cells caused death.