1.13 Update

You have to switch manually to 1.13

Afaik the world system will change in 1.13. Your world will automatically be converted to the new system as soon as you start your server with 1.13 for the first time.

However I heard that a world converted 1.13 can't be rolled back to 1.12.

I recommend you to create a backup before moving to 1.13 in order to change back if 1.13 doesn't work properly or the conversion failed.