A Macs startup drive can be changed during system boot by holding down the OPTION key, start holding down Option the moment you hear the boot chime sound and while the screen is still black, this will bring up the boot drive manager. From this startup boot disk menu, you can use the mouse or the arrow keys to select the drive or partition to boot from, and the hit return to startup from the choice.
This allows for a per-boot one time selection, and selecting a boot drive from this menu will not change your default boot drive. For future boots, you can either continue to hold down the Option key to access this menu, or you can change the default startup disk directly from System Preferences if you’re looking for something more permanent.
We’ve used this many times to change boot OS versions with dual-boot configs, from Mac OS X 10.6 to 10.7, 10.7 to 10.8, and most recently with 10.8 and 10.9 Mavericks.