To change your master ball into a poke ball you must first get at least 3 poke balls and once you done that get into a battle. Weaken the wild Pokemon by 40 percent [by estimating]. Now chuck a Poke ball at the wild pokemon, when the Poke is in midair hold down and B button, if you done this correctly the poke ball should altomatically change into a master ball.

NOTE: If you use the cheat on a very strong pokemon the cheat won't work.