Okej, jag ska svara på vad vi alla har varit ganska säkra på ...
Berättelsen du letar efter är "Desertion" av Clifford D. Simak.
A later tale tells of a research station on the surface of Jupiter. (This story, first published as Desertion in 1944, was one of the first stories about pantropy.) Simak's version of Jupiter is a cold, windswept, and corrosive hell where only advanced technology allows the station to exist at all. A scientist is accompanied by Towser, his tired and flea-bitten old dog. But there is a problem. Men permanently transformed to survive unaided on Jupiter's surface leave the station to gather data and inexplicably fail to return. Finally, the scientist transforms himself and his canine companion into the seal-like beings that can survive the surface. They leave the station in their new form and experience Jupiter as a paradise. Towser's fleas and irritations are gone and he is able to talk telepathically to his former master. Like the previously transformed station personnel, the scientist decides never to return.
(från Wikipedia )
Det bör noteras att, medan "Desertion" finns i många antologier där ute (jag läste den först i The Ascent of Wonder: Utvecklingen av Hard SF ) Det är en del av Simaks egen antologi City (1952), som använder "Desertion" som en del av en övergripande historia om människans långsamma, frivilliga nedgång. Jag rekommenderar att du ger hela antologin en läsning, men självklart kan historien läsas ensam utan problem.