Convert an already recorded stereo track to mono in Garageband
This was driving me mad, but I now use an very smooth way based on JHerran's answer. It has the virtue of only requiring Garageband and is quick.

N.B. I assume your track has a single region: if not you will have to shift-click to select all the regions in the track that you want to convert.


Click on the track that you want to convert. The single region should automatically be highlighted.
Command-D or >Track>New Track with duplicate settings to create a new track
Command-V to paste the original regions to the new track
Shift-Click on the original region(s) to highlight it again
Command-J to join the regions, select "Mono" for the mix-down
The mix-down will replace the original stereo track - you can now click on the temporary track and Delete it.