You can change the character's alignment by finding the right bit in the character file (hex offset 10A in Champions), but it doesn't unlock the evil spells. For that you have to go to the address bits for each spell and set them from 00 to 01.
Example: In Champions, the curse spell address is at offset 64. You'll notice the two bits surrounding 64 are 01's for bless and detech magic. If you open up a cleric character save file, set bit 64 from 00 to 01, save, and then start the game, when you go to memorize spells, you'll see curse on the cleric spell list for that character.