I'm working on something like this right now. It's with a character who has as a major trait in canon of being a fish out of water, and I'm working on a fork-in-the-road au where he'd be comfortably in his regular waters. It's weirdly the opposite of your scenario - he's matured a good bit in canon. Basically the main thing I'm trying to do is lean on the recognisable parts of his character that I can still use, and hope that that keeps the link obvious. Sense of humour, etc.

Not sure I have a complete handle on it, though. Good luck to you.