I backed up all of my own files except the jars apparently (because when you download every one they added up, and I didn’t do that when the servers were at stake). I even had a launcher still logged in but none of the files will download now. Prism is lame (but understandable, I guess) in that it just says “contact microsoft support if you didn’t migrate” or something like that, but you can just copy over accounts.json from polymc to use an offline acct. Though a few mods I’ve tried don’t work (and I feel like mod discoverability might not be the best?).

Also a small bit not directly in reply to you: I’m pretty sure this is actually the second migration too, at least for accounts that were started on the minecraft website (username–>email login+mojang acct). But of course searches only give info on this one.