There’s no guarantee that we can’t keep existing worlds live for original backers in some shape or form, of course that requires effort since those worlds would need to be kept separate from the rest of the MMO which may not be something we need to implement for initial launch otherwise.
There are some features not yet implemented which will certainly make existing worlds ‘strange’ too, eg:
World regeneration: The design we currently have in mind would mean that unless we have backups of the original data the worlds were made with at their origin, the world would end up ‘regenerating’ to the point in time at which that feature went live (Eg existing houses built by players would get regenerated if modified outside a beacon <-- wuuut?). Trees and ore veins (not even all the worlds live have ores at all!) would not regenerate in the same way they would on new worlds since the information required to collapse trees when partially mined, and regrow them from seed etc would not exist in the world data.
Creature spawns: Though we don’t currently have a finished design on this, it is likely going to be tied to things like biome information which again, current live worlds have no information about so when creatures go live, we would be able to put them into current worlds, but not in the same shape/form the feature would behave like in new worlds that have all the extra information directing the spawning.
Current live worlds also have no monuments/temples etc or areas (and like creature spawns information driving) things like protectors/titans to exist in the world.
We have (non-finalised) plans of doing wrapping worlds, which again would be quite strange with existing worlds as we’d need to simply define a size for that world and discard any chunks outside that area, letting the rest of the chunks wrap (badly) since they were not generated in a wrapping world.
Things like new plant types or foliage types (seaweed, nettles, more flowers, etc etc) do not exist either in game or in any of the current worlds, so if any new plant life is added it would not exist in any of the current worlds. This was already the case when it came to the new grass foliage; the migration for existing worlds did not have enough information to do a realy good job at it, so you do not get any of the nice flowing gradiented pastures you get in newly generated worlds.
Etc. etc 