@emacsen
Do you want to change your username, or just the address of the (web interface) of the instance?
I would have thought if you do not want/need to change the username, then it just works with a redirect in https://emacsen.net/.well-known/host-meta to https://mastodon.emacsen.net/.well-known/host-meta but, I also would have thought there's people better versed in activitypub than me on the thread.
Your post makes me realise how little I have poked around in the mastodon backend. I didn't even know for example how the links to toots (on the minutes/hours/etc ago) top right are formed, but I see they are in the uri column of the statuses table. So, maybe you could do some fancy redirection on your new emacsen.net to urls that should go to mastodon, like /users/emacsen/statuses/*. I don't suppose that would conflict with any of your new content.
@keith
I want to move the instance so I can use the web site (emacsen.net) for multiple things, and have mastodon or social.emacsen.net for Mastodon.