//...if the person previously known as Eric suddenly wants to become Erica and use she, so why does it upset you so much?//
Doesn't upset me at all. Provided that Erica with a penis doesn't demand that I call him a woman.
People who thought that they were Napoleon used to be locked up. Not a policy I agreed with (unless, of course, they're marching into Russia with half a million soldiers).
Depending on context, and as a matter of civility or kindness, I'd readily address Erica with his penis as a she, or the "Napoleon" as empereur. But I'd prefer that to be a matter of my discretion rather than a legal dictat.