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.

You get the most flexibility by hosting your own instance since you get access to all instances and all communities and you won’t be defederated from unless you act in a toxic way.

The only draw back is that you have to subscribe to each community individually since you won’t have a “local” timeline, especially not one as populated as lemmy.world.

But if that’s not a problem then self hosting, or the next best thing, choosing a small non-polemic instance is the way to go.

Mostly it doesn’t matter for the person using it*, so you can just pick one that isn’t overloaded to start. But…

Ways it does matter:

• Your instance’s moderation policy and actions. (including what content is allowed/disallowed, how they deal with harassment, etc.)
• Server reliability. This can change drastically if a lot of people join at once, as many Lemmy sites have discovered this week! (I believe Lemmy.ml and Lemmy.world have both upgraded their hardware in the last few days to deal with this!)
• Admin reliability. This is harder to tell up front, but it’s worth taking a quick look at whether the admins seem to be active and responsive, whether they seem like they’re in it for the long haul or if they’re experimenting, etc.

Switching is sort of easy in that all you have to do is create a new account somewhere, and you don’t need to tell your followers because Lemmy doesn’t have user subscriptions (though someone could follow you from, say, Mastodon)…

…but it’s also not easy in that Lemmy doesn’t have tools to export/import your subscriptions (yet?) so you have to add them to the new account manually. And moving your posting/comment history isn’t something that’s doable at the moment, either.

What I did when moving from lemmy.ml to lemmy.world was put the old/new accounts in each others’ bios and add “Old Account” to the old one’s display name. I’m not too attached to my post history sticking to my profile.

*I think it matters a bit more for where you set up a community, on the basis that an instance focused around, say, history would be a better place to create an archaeology community than one focused around FOSS. Though you might want to cross-post articles about free software used in archaeology!