You wouldn’t lose the posts you’ve made, rather the posts you’ve made will be hosted from one instance, rather than all of them.

You’re a lemm.ee user, if you upload to a lemm.ee community nothing will change.

If you upload to another community, then normally you’re post would be uploaded to lemm.ee. This would then be federated, and users from other instances would load the same content, but it would be delivered by their own instance.

The change refers to things beings hosted only in your host instance. Thus, a lemm.ee user may load content from a lemmy.world server more often. Normally, lemm.ee would copy the content to its own servers and direct its users to that, but now everything will go to the host instance.

The only thing I’m not sure about is who is the host instance? My understanding is that the host instance is that which the user belongs to. Thus, if a lemm.ee user posts to a community in lemmy.world, techincally the federated host instance is still lemm.ee - it’s about the user, not the community. But with all this I’m not sure.