Star Trek: Deep Space Nine changed everything about the "Trill" species in order to make for better TV.

When introduced on Star Trek: The Next Generation, they were your standard bump-on-forehead alien (kinda very ugly look). They couldn't use the transporters or it would kill the symbiont, and the symbiont completely subsumed the host's personality, as if the host was just a shell for the symbiont to inhabit. In addition, noone outside of their people knew they were a 'joined' species as they apparently weren't part of the Federation but just some aliens.

For DS9 my guess is the makeup didn't look good on the gorgeous Terry Farrell and having no transporters would have severely limited what kinds of scenes her character could participate in. They also retconned them to have been not only in the Federation but in Starfleet.

This led to a much more interesting character and backstory for Dax.