It was a series of 3 or 4 books. The crew of a ship were on a mission to recover something ( rings I think) from different planets. Humans had been genetically altered to live on each planet. Volunteers were permanently altered to blend in on the planet to complete their missions.
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.
- Lilith: A Snake in the Grass
- Cerberus: A Wolf in the Fold
- Charon: A Dragon at the Gate
- Medusa: A Tiger by the Tail
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.
Jack Chalker has a series from the late 80's called Rings of the Master that includes modifying humans to fit into the societies of aliens to find five rings.
The books are:
- Lords of the Middle Dark
- Pirates of the Thunder
- Warriors of the Storm
- Masks of the Martyrs