For one thing, had that person transitioned during our relationship it wouldn't have been a dealbreaker. I happen to be bi, and I could have easily shifted to introducing a romantic partner with a new term. I understand that wouldn't automatically be the case for everyone else in this kind of situation, but from my perspective the person's gender presentation was not something that would have ended the relationship on its own. (The relationship just ran its natural course, and we parted on good terms.)

But, putting that aside, I'm still content to treat my previous perspective on that relationship as a misunderstanding at the time that has since been corrected, and to respect that with a retroactive change in description. My ex who I thought was a girl turned out not to be, and that's completely fine. His transition is something I celebrate for him, and while part of that celebration affects my own story it's still his own story first and foremost. That's not something I wish to disrespect with inaccurate terminology when referring to the time we were together.