What happens?

 

Sorry if this has been done before but what happens in tournament games whenever a pawn reaches the opponents other side? Do they bring out a new piece and replace it or do they just have to remember that that certain pawn has the power of Queen, Rook, Bishop, Knight?

Well, usually there is a spare queen that you can use, or you can use your queen if it has been captured. Just remove the pawn from the board. I can't imagine when the player would rather promoter the pawn to a rook instead of a queen, but I can see some situations where you would want a knight instead. If you don't have an extra piece, and the piece you want to promote it to has not yet been captured, then I suppose you would need to remember that pawn has been promoted. But that situation is very rare. Usually in the end game, there are many pieces taken, so the one you want is almost always available. Like I said most tournament sets, have an extra queen for each side.