The CPU cannot be changed out because Apple solders them to the motherboard, and that is if you can even get inside the damn thing. This goes for the aluminum body Macbook pros also.
If you consider yourself gifted and you can unsolder about 200 pins and resolder the new CPU in, go ahead and upgrade but 99.999999% of the populace cannot do this (Probably not even repair shops) So if you're the 0.000001% that is gifted to change a macs CPU, go right ahead, XD
The best thing to do is just buy a new mac
Or if you have the Apple care plan just try to get them to replace it :S