This question already has an answer here:
I read these novels in the late 80s or early 90s. In each book, they had to find one of a collection of things, which may have been rings, to do something or other like save the universe or whatever. They had the technology to transform someone to another form to match whatever was the dominant species of the current novel's target planet was, but for some reason, I think they could only transform once, so much drama ensued in choosing who would be the one transformed.
As another clue, my grandma bought me my first book in the series (not the actual first) because it had "Pirates" in the title, because she thought I would like a book about pirates, not realizing it was a deep-space science fiction series. I did like the series as an early teen, but I can't remember what it was and don't have my childhood library available to me.