I read a book that I had checked out from a public library in the 1980's. I was probably 10 years old or so. It was a fantasy book where the main character was turned from a man to a woman (or vice versa) through magic or sorcery. The setting was in more ancient times... Unfortunately that is all I can remember, but I remember loving the book! Has anyone read any titles that match this vague description?
Commitment Hour by James Alan Gardener, fits most of your info but it was published in 1998.
Even though it takes place on a post apocalyptic Earth, the community resembles a 17th century village.
When each person reaches 21 years of age, they must choose a gender (male, female or hermaphrodite)
The Tamir Triad may be that book? A young girl is saved from the usurper king by turning her into a boy. When she comes of age she has to turn back into a princess to claim her throne. It's a trilogy by Lynn Flewelling.
PS: oops this book was published in 2009, so it's probably not it.
This doesn't fit your timeline, but the Elminster Series written in Forgotten Realms D&D fits the rest. Elminster is changed from a man to a woman by the God of Magic for a period of time.