There are plans to support 3rd party identification (such as an E-Mail address or a phone number) and use that as a basis for looking up users across the network, but I don't think it is currently useable. Account migration was brought up recently in the chat room, but it is not defined anywhere in the spec or reference implementations AFAIK. I agree that these are both important features, but I wouldn't worry too much about them unless they are left out of the 1.0 spec.
In the meantime, it's not like you can migrate your Signal, Telegram, iMessage, or even Gmail/Hotmail accounts. I think Matrix needs a few more client/server implementations before the spec can't truly be set in stone.