Okay, if you see a code in Lower Case letters, its the same thing as Upper Case. Its just that the people didn't bother typing the code in with all Caps instead of lowercase. (To answer you're comment above), the difference in Caps and lowercase is nothing. Use the Letters and Numbers normally.
Don't worry about it and type in the codes normally.
Though, Action Replay is not recommended and I suggest you don't use it cause it will mess up and "destroy" your game.
Hope this helps! ;D