2) Every piece of data in the system will be available via the API including negotiated app and follow credentials, moving will consist of authorizing an importer app to have read access to everything, and then pushing a post that tells all the servers to check the profile again for updated entity and server details. It should be a very simple process.
After all, there's a reason social services are centralized on today's Internets. The reason (IMHO) is that the Internets since 1992 or so have been an antisocial network, and anything worth attacking that lacks a centralized defense command is rapidly overrun by digital Huns. For instance, SMTP exists today because it existed before eternal September, and being valuable was (barely) defended; but if it didn't exist as a legacy from the old, social Internet, it would be very difficult to create it in the new antisocial one. If not impossible.
I mean, it's certainly not that some of us rotting old neckbeards weren't using finger and talk on the firewall-free Internet in 1989. So we know how cool it would be if some bright young whippersnapper could solve teh problem...
(2) This is useful but inevitably imperfect, as forcing every interlocutor to equate the old and new names is of course impossible. Eg, HTTP redirects make it possible to change your DNS identity - but hardly trivial, though the redirect itself is trivial.
And of course it's a process that your existing host could easily frustrate, though that would be very ill-mannered. Not saying there are any perfect solutions here.
1. Just because it is allowed by the protocol doesn't mean any given client needs to pay any attention. Just like email, I can filter out any messages from people not in my contacts. I may choose not to and instead run each one of those messages through a spam filter. In this respect it really seems no different than email. Individual clients/servers can choose to be as strict as they like (but servers are servers, and they are sitting on the internet, so spammers can see them and send messages that will be ignored if they like).
2. Since connections with most of your contacts are theoretically maintained so you can push out new data, updating is more akin to propagating a new ip through the DNS system than using a redirect. Yes a DNS server can misbehave, but that can only screw up a network of well behaved servers for so long.
There must be some reason we haven't seen successful new decentralized service protocols on the Internet since the early '90s. I don't know of a more obvious one.
You can see the issues with StatusNet and spam:
https://www.google.com/search?q=statusnet+spam
2. The problem is that contact names propagate outward from the master state where a push will update them. For instance, they get written down on business cards. They also get cached, imprudently but inevitably, in forms that are still digital but don't update properly.
Imagine a protocol that you could use to update your email address this way, and you'll see the problem. In theory, you could design a special SMTP message that would cause all clients to update their address books. In reality this would scale quite poorly and be quite unreliable, leading people to avoid it, leading it to be even more unreliable, etc. Of course, your chances are much better with a bright, shiny new protocol... but still.
This would be especially bad if the server you're using is hacked and they stop everyone from leaving :/