I'm with sparrow, if you are fully perceived as who you are, there shouldn't be a valid reason to bring up something so personal unless that person has to know (like a doctor, sexual partner, etc.) If you know for certain that they would be supportive, you could reach out about it, but that's your choice. I haven't told any of my friends because I'm not really on a deep personal level with any of them. The only people who I tell in rare instances are family members who visit and ask where birth me is or those who need to see legal identification.
People do sometimes sort of change they way they think about you once you tell them you're trans, its sort of like they try to picture you as the opposite gender or something like that. It applies to anyone really, like if people say a cis woman like Michelle Obama is trans, their brains will sometimes automatically try to picture her as a male and have that image in their head. But yet again, you've said most people who do know you're trans have forgotten, so you must pass very well since that means they see you 100% as the gender you are.
