Ok, so, I edited a world in MCedit 2 to reduce the file size and clear some distant chunks. I then opened the world in 1.12.2 (Latest Release), and it loaded just fine. When I go to upgrade that world to 1.13 pre-release (3 and/or 4), I get the "Loading World" text and screen, but it never actually loads.

I've tested this over 12 hours, just leaving it to attempt overnight. The only way to cancel the loading process is to kill the Java process through task manager.