any way to switch from lemmy.ml to lemmy.world or Kbin or Beehaw?

You need to register in the instance you want and then add the new account to the app

There will likely eventually be a way to migrate your account just like mastodon, however kbin is a different platform altogether and you would need a new account there.

Nope, so far the app will log into the last account you've used.

Someone made this cool tool for Mastodon that lets you switch. Maybe someone will do one for Lemmy soon.

https://github.com/stefanbohacek/mastodon-tools

The default instance is your instance (local). In the setting you can change this to all. Under user settings.

I'm sure there are a lot of "previous state" saves it's missing to make a better experience. I always have to reset to Subscribe: New Comments for the best experience.

As far as I'm aware, the instance or platform you register in is the one you get as your hub, even if they are federated together with others.

I don't know if you can migrate your account between instances, but my recommendation might just be to register a new account with the instance you want to be your "home" instance.

you have to register in the website of the instance you want to use, then search it in the drop down menu (if you can't find it just type the domain name example: "lemme.ee") and insert your credentials

So can I log into Jerboa with my kbin.social account or not?
Edit: Turns out I can't. Jerboa seems pretty useless to me if I can't filter out the unwanted instances.