*** Update: To persist this change between machine restarts, see this post ***
1) Save this script as monitor-switcher.sh:
#!/bin/bash
# Author: Andrew Martin
# Credit: http://ubuntuforums.org/showthread.php?t=1309247
echo "Enter the primary display from the following:" # prompt for the display
xrandr --prop | grep "[^dis]connected" | cut --delimiter=" " -f1 # query connected monitors
read choice # read the users's choice of monitor
xrandr --output $choice --primary # set the primary monitor
2) Make executable with:
chmod +x monitor-switcher.sh
./monitor-switcher.sh
All credit to the article here.