Wario Land 2 was origionally released on the Gameboy, being compatable with Super Gameboy, in March 1998.
The game was later re-released on the Gameboy Color, with the cartridge being backwards compatable with the origional Gameboy and the Super Gameboy.
However, there was a difference in thr compqtibility.
The origional cartridge (.gb rom) can be switched between playing on a gameboy or on a super gameboy any time, with the save file being compatable.
However, on the GBC cartridge (.gbc rom), the save files are not compatable.
So, if you start playing the game on a certain mode (GB, SGB or GBC), you will not be able to switch to a different mode and continue playing. Doing so will give you an error message, and prompt you to erase your save data.