Effect: David Blaine performed a version of this trick on his first  Street Magic TV special where he gave his spectator one card, took another card and it visibly changed in front of the spectators eyes to the card that was previously held in the spectator's hand, but when the spectator turns over their card, they see that it is actually the card held by David seconds before !!! The two cards appear to have magically "warped" between the spectator's and magician's hands. 

Preparation: Before the trick starts, take two of the same cards from two different packs. So now you have two different cards duplicated. Lets say you chose the 7 of spades and the queen of hearts. Prepare the deck so that you have on top, the queen of hearts, then the 7 of spades, then the queen of hearts then the other 7 of spades. 

Secret: Start off by double lifting the first 7 off, when the top card behind it is really the queen. Put the double lifted cards back down before taking the actual top card, the queen and without the spectator seeing it, place in their hand and put their other hand over it. This is where the difficult part comes in. Double lift the next 2 cards, with the queen looking as if it is only one card but the second queen is secretly hidden behind it. As you turn the 2 cards over, put them back down with the queen facing up. Now turn the queen back over after drawing attention that you have the queen, but as you turn it over, secretly leave the 7 facing the wrong way round. Now double lift the 2 cards again( one facing up, the other facing down behind it secretly). Now put your first finger on one corner of the face up queen and your thumb on the other. Start to shake the cards as you press your fingers together, forcing the cards to flip over, (this is the snap change) so it appears to the spectator, that the queen has magically changed into a 7. Then put the now face up 7, face up on the deck to hide the queen and ask the spectator to look at the cards in their hands, where they will find that the cards have switched.