The game crashes when trying to upgrade a world from version 1.12.2 and earlier. I won't provide the crash report for privacy reasons, but you can reproduce the issue yourself and obtain the crash report.

Steps to reproduce

1. Start version 1.12.2
2. Generate a new world with the default world type
3. Launch the latest version of the game
4. Attempt to upgrade the world by entering it

Observation: The game crashes.
Expectation: The game should function without crashing.

Launcher:

The game crashed whilst exception loading chunk
Error: java.lang.ClassCastException: class com.mojang.datafixers.util.Either$Right cannot be cast to class com.mojang.datafixers.util.Pair (com.mojang.datafixers.util.Either$Right and com.mojang.datafixers.util.Pair are in unnamed module of loader 'app')