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 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 I a few mods I've tried don't work (and I feel like mod discoverability might not be the best?).