special_level.dat and special_level.dat_old
In the April Fool's version 20w14∞, in order to protect normal world saves from being loaded, a differently named level.dat file was used.
Obtaining
In 20w14∞, special_level.dat and special_level.dat_old are used instead of level.dat and level.dat_old, respectively. These normally can't be added to a pre-existing world save, due to level.dat files being created upon world generation, but the use of world save insertion allows all the world files and folders to be preserved while still generating the special variants.
These files can normally be updated out of 20w14∞, assuming the level.dat files are still present in the world save. (In the case only the special level.dat variants are in the folder, the world can still be updated, while still preserving the special variants by using world save insertion to update out.)
These special variants can also be used to preserve April fools exclusive features within them, since the data stored within them cannot be changed in another version.