Effect: Ask the spectator to choose a card and place it on top of the deck. Show them their card and place it in front of them. The card magically changes to a different one at which point you get them to place it back in the deck upside down. The wrong card then turns into their original card.
How It's Done: The standard 'choose a card' routine is done and at some point you need to palm a card. Get the participant to place the card on the top of the deck. I find if the deck is a little messed up at this point it is easier to place the card you palmed back on the top of the deck. At this point their card is the second one from the top so to show them their card you need to do a double lift. This will show them their card and take any suspicion off of a perhaps slightly flawed 'palm drop' of the top card. Draw the card off the top of the deck and place it in front of the spectator. Now is when you have to be quick and clever. Get the spectator to put the card on his head or something to distract them for a second as you need to move the top card, their card, to the bottom of the deck upside-down. To do this is actually easy with practice, it is just a matter of rolling the deck while a thumb keeps the top card in place. What you want to have happen here is to have the deck upside-down with the spectators card on top face down giving the impression that the whole deck is upside-down. Then get the spectator to place the card back in the deck upside-down. Now what you need to do is easy, just bring the deck to your side as you explain that your going to change the spectators wrong card for the right one. Bring the deck up but bring it up so the real card is on the bottom upside down and deck is right-way up. The only upside down card in the deck is the spectators card and it is on the bottom so do a little shuffle. Now find the upside-down card and look at the amazement on the face of your spectator.