well, all i can say is that in some version of windows, with some kind of sound card, there is an option called "reverse stereo" which will do what you want
sadly, i cant find this on my machine so it is probably specific to a particular sound card..
the only other option i can think of is to get a stereo to mono splitter so you have the individual left and right channels and then get a mono to stereo converter and plug the left channel into the right and vice versa...