I’ve been listening to Orlando, by Virginia Woolf, and had heard a bit about it so I knew the main character transformed into a woman–but I had expected it to be a sort of reincarnation thing. As it happens, he just wakes up one morning to be a she with no explanation given. And, I found that no explanation was needed. The ‘how’ of it isn’t what the story is about, so there is no need to give it any time. It happens, and the reader (hopefully) can accept it and get on with enjoying the story. Any pause to try to explain an ancient magic curse, or a special potion or an ancestor’s ghost or any other such thing is just a distraction, and would feel forced in.
What I take from this is: trust yourself. Don’t feel you need to explain everything and have logical continuity for everything in your writing. If your story isn’t about the how of a thing, then don’t feel the need to include any but the vaguest explanation–the reader won’t mind! I know I don’t.