Anyway, if you run Spigot, Paper or if there is any other that can use Worldedit, you can restore chunks in your world to the aquatic level, if you have that version...

Want to restore? Simply type: 



If you run into troubles, they can vary in:

Roads rails builds etc.. gone!

Crashing your server

Kicking you from game

Just make sure you have a backup from where you can restore sections in the world, then you are ok! If this happens and you have not noted where you were, you might find yourself in a bit of trouble: make sure to have a backup to restore that part and then try a smaller selection. No worries here, go back in, do not move at all and just type /back (if you have that command configured)

 This is just one example of the difference, left is newly regenerated and right was the old part of that world. Somehow there was ocean instead of land....

2019 09 29 20.22Take notice of the updated action from 1.17 to 1.18, if you proceed doing this on an existing world, do remember that //regen calculates totally different map regeneration, it is a MOJANG thing that made this possible.
Instead of doing it their way, they could keep the worldseed for the existing world the same and only act upon the lower - newer- part of the world, but no! they decided it was impossible to keep it at that.


You with your existing worlds out there? BAD MOJA! That is Mojangs new slogan.

Want a work around? There is almost none, going temporarily back to 1.17 corrupts your world, want to only use a small part that needs regenerating? Yep, you guessed right, make a backup server with the plugins and the worldseed in 1.17 and not copying the world, just note down the coords and go there. That is what I should do.

Pity actually that this is not the first time that server owners had to scratch their heads over this idiotic thing, they only seem to be interested in the client side.