I was trying to transfer an entire world to another. Switching the regions worked for the Overworld, Nether, and End. I could switch the data like maps and raids, but my problem came when trying to switch the playerdata and stats. In 20w12a, if I try to switch playerdata or stats, the game crashes, so I went back a snapshot. In 20w11a, they just reverted back to empty files for the new world, but no crash.
Not to mention the level DAT files and session lock file, if I switch those with another world, that world gets removed from the launcher.
Don't tamper with world save files if you don't know what you're doing.