Hi Aternos Requests Team,
I think you should add the following plugin..
Name: WorldUpdater
Link: https://www.spigotmc.org/resources/worldupdater.66796/
Description: This plugin seems vital for the transfer between 1.13 and 1.14. It allows loaded chunks (You can save chunks with WorldGuard) to be removed from the world in 1.13, and then the world can be loaded in 1.14 with the new format.
Thank you,
Charles