Each instance will have it's own rules and has the capacity to include or exclude other instances.

For example, you want a work friendly instance that does not index NSFW, gambling, drug use, or hate speach. Chances are that you can find and join that instance. You'll see posts from instances that aren't blocked and your posts will be shared with instances that have not blocked yours.

If you want to switch, then you need to create a new account on the new instance. You can maintain accounts on both instances, but they would be separate accounts.

Additionally, if you want to be ruler supreme, and are willing / able to do the tech work, you can set up your own instance that only you and those you invite are able to join. This instance would join the Federated community to share content based on settings.

I know instances can block pulling in content from other instances, but I am not sure if they can block broadcasting content.