In the episode "The Host" where a sort of proto-Trill species debuted, we have Odan, a male host (Who Dr Crusher was romantically interested in) dying and the Trill passing on to a female host (after spending a few days in Commander Riker), also now Odan.

At least they had the scene where Crusher notes that she would find such a relationship (where the one who is loved would retain the knowledge of the past bodies, but might change body and gender) impossible for her. She doesn't judge Odan (which would have been insnaely crass) but herself for not being able to accept that kind of relationship, and expresses a hope that someday humanity might find such a relationship viable. Since such a thing can't happen in human society terms, I'll give them a bit of leeway on that one.