Starting with vanilla Minecraft 1.13, an option to "Optimize World" became available, basically allowing you to upgrade the full world to the current version associated with the server.

To enable this on the server-side, you can modify your .tweaks  file and enable the following option. We highly recommend that you take a backup prior to enabling this.

## Force Upgrade (1.13+) / Optimize World
#
# Forcefully upgrade the full world prior to the start of the server.
# This option should be disabled after the first successful run.
#
## It is HIGHLY recommended that you complete a backup prior to enabling this!
#
force-upgrade=false

Change the value of force-upgrade  to true, save the file, and restart the server.
Your server will display a message stating it has started: Forcing world upgrade! 

Once complete, we highly recommend you disable this option after the successful run in order to prevent any potential issues with the server in the future.

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