I remember reading (some) of a series of novels in the 1980s (or earlier, I read old books then), where the protagonists were transported to a world that was divided into different habitats suitable for different types of intelligent species. The habitats were not necessarily compatible (some Earth-like, some suitable for chlorine-breathers, etc). People transported to it were transported into a body suitable for the habitat, even if it wasn't the species they were originally.
The main plotline, if I remember correctly, involved the hero trying to find out what was going on, and why this was happening. I don't remember many details.
The author wrote a lot of books where body changing was a major feature. One stand-alone novel featured a pair of heroes conscripted by an alcoholic demon to collect items from alternative worlds, to give the demon the power to destroy an Earth-killer asteroid. They were "injected" into the minds of their counterparts in the alternative worlds, for good and for bad. There were at least two other multibook series with body-transforming protagonists.