It's been a few years since I was on here, but some things have happened and I no longer want to be associated with my ex's surname. Is it possible to change my username?

Users can't but admins can; but even that only affects posts going forward, leaving the existing post history (and, especially, post quotations) unchanged.

Thank you for the response.
Would my old comments share my signature if I put my new username in it so that other users can still find me?

All posts from the same account have the current signature shown - e.g. my very first post here.