Last I heard, Mastodon didn't support migration between instances while still keeping followers, etc. Instead, you have to essentially start from scratch with a new account on a new instance. This is a serious problem because it strongly discourages exodus; Twitter itself shows that people will put up with a lot of nonsense before leaving to alternatives because they've cultivated a brand and a following and don't want to lose it all. Moderators of instances are not a static thing; like any political system it shifts with whoever is in charge over time. An instance that seemed friendly to a user initially can over time become more oppressive to them, but the user has invested so much into that instance that they would have to leave behind that they'd put up with it when they shouldn't have to.