the only difference between the Athlon XP and MP is that the MP is certified to run in Multi Processor systems, and the XP is not. Besides that the price is different, and the Athlon MP comes without the bridges cut so its easier to unlock that CPU.
So if you got an Athlon MP at the price of an XP there is no need to return it, or exchange it for the other CPU. You might want to sell them if you are able to get them cheap though :smile:
Welcome to the end of your life Mr. Intel CPU. Don't worry I promise its going to hurt.