Changing track from left to right speaker

I recently acquired a Macbook with Garageband '09. I made a recording with two mics, one recording a guitar, and one recording a drum kit (They were both recorded at once on the same track). Since I had set the input to stereo, the guitar (mic 1) played through the left speaker, and the drums (mic 2) played through the right speaker. I would like to have mono sound, but I still want to use two mics to provide better clarity and prevent spill.

Is there a way to keep the track I have (guitar=left/drums=right) and perhaps duplicate it and switch channels from left to right creating a mono sound? Basically one track would be guitar=left/drums=right and the other would be guitar=right/drums=left. Or is there perhaps a way to switch a stereo recording to mono once recorded?