It’s never been possible to change the full diaspora* ID (
firstname.lastname@example.org) – mainly, I think, for reasons of federation breakage.
However, once the account migration feature has been implemented, this should no longer be a problem. If it’s possible to change a diaspora ID to one on a different pod – and, I assume, the username doesn’t have to be the same on the new pod as on the old when migrating – it should technically be possible to change the username while staying on the same pod without breaking federation.
In the worst case, this would be a kind of ‘within-pod’ migration, although hopefully there will be a solution that is simpler and costs less in resources.
Those are my thoughts and hopes as someone who really doesn’t understand the technical side of things – now, technical people, shoot me down!