I got curious so I start digging into how mastodon do it. It’s more like a hack, really. Mastodon uses WebFinger to resolve user account, so when you change domain, you can leave the old domain up so your federated servers can still resolve your users and realized the domain has been changed and update their federation data. But it turns out you can’t exactly retire the old domain either because it’s still tied to user account internally. So if you lose control of your old domain, you’re probably as screwed as fmhy.ml.