If I were to buy premium and use it for a while, what what would happen to the premium page types I've already used and the universes I've made if I stopped paying/went back to the starter pack?
I'm really confused as to what would happen?
They would still be there and you can still keep editing them as needed (I think, I'm pretty sure)
You’d be able to continue editing the pages you made, but you wouldn’t be able to make new ones.
Just to clarify, all of the above is correct: any page you create is yours forever, and you can edit/view/share it no matter whether you have premium or not. You just can't create new premium pages without an active premium subscription. :)
Happy worldbuilding!