Impossible Card Transposition

Two cars are selected from a blue deck and put face down on the table with a red deck (in its card case) on top of them. The magician lifts up the red deck and drops it back on top of the two cards which magically change into red backs. Then, the red card case is opened and the deck spread on the table to show two blue back cards among the others, these two cards match the selections. The transposition is complete.