One thought: changing your email address might require entering your password as well.
@mcfate @th3j35t3r : Just tried that. Still can't get in. Thanks for the suggestion!
I'm out of ideas at this point. It might be immutable, but I don't see why it should be.
@mcfate @th3j35t3r : Thank you for trying to help!
@JoeOTAir
Hm.
Any ideas, @th3j35t3r? It may well be used as a database key, so it could be fixed at account creation, like the user ID is, I suppose.