the plot as best I can remember: Middle aged woman (spy?) was set adrift/banished from earth in a space ship. Saved by near-human aliens she ends up undergoing a special surgery/process to take the place of a teenager (princess?). She is captured by a group that conditions young women to be sex slaves and a side effect of not being exactly the same species is that her older mature competent mind is not in complete control of the young body she inhabits. This makes it difficult for her to escape. It was surprisingly adult content for a 1984 high-school book club book.
It isn't a perfect match, but it does sound like one of Chalker's "Dancing Gods" series. He does a lot of things where peoples bodies and minds are changed at the whim of heartless power mongers.