I believe I read this one before, years ago. It's by far my favorite Simak short that I've read so far.
On Jupiter, an experimental program is in place to transpose men into the bodies of Jovian native fauna in order to allow people to go out into the hostile environment. The procedure seems to work perfectly - but something is going wrong. So far, the first four test subjects have gone out into the wilds of Jupiter - and have not returned.
The head of the program may have no moral option but to change tack. ( )