In Minecraft 1.13: Update Aquatic, Mojang have removed the ability to open customized worlds from older versions of the game.
Of course this is a big problem for players that have an existing world and want to upgrade it to 1.13.
Here is a video showing how to upgrade a 1.12 customized world for 1.13 using the tool on this page. We've updated how this page looks to give you more help, but the same drag n' drop box is on this page a little further down.
🚨 BACKUP YOUR WHOLE WORLD FIRST - USE AT YOUR OWN RISK 🚨
Have you seen this tooltip when trying to open a 1.12 customized world in 1.13?
This world is no longer supported and can only be played in 1.12.2
This means you have a customized world and it can't be automatically played in 1.13.
If you click on the world to load it, then you see this message:
Unsupported world! Unfortunately, we cannot support Customized Worlds in this version of Minecraft. Please downgrade to 1.12.2 to play this world. We are sorry for the inconvenience.
Don't worry, we can fix this!