I wrote a thing[0] about Mastodon a while ago. Currently there is no way to migrate an account, but that's being worked on actively, and progress is being made. You can already export your data, and who you follow, block, or mute.
Ideas are being discussed as to how your followers can follow you when you move, but that's tricky given the entire concept of federation.
This is explained on the page I wrote[0], but if anyone on your instance follows person X on another instance, toots from person X turn up on the federated timeline on your instance, you can have a chance to see those toots. Similarly, if someone on your instance boosts a toot, it turns up on your instance, so you can see it and decide whether or not you want to remote follow that person. This the network of connections grows, albeit slowly.
And yes, you can comment on threads on other instances.
Hope that helps. You can always join an instance with a throwaway account, try it out, and if migration does come to life, migrate. If not, start afresh on the instance of choice with a new account. Come find me here:
https://mathstodon.xyz/@ColinTheMathmo
[0] http://www.solipsys.co.uk/new/ThinkingAboutMastodon.html?HN_...