How to Switch Startup Partition without entering password
You can press the Alt-Key on startup to choose from the different booting devices... So you can just restart the Mac and when it "chimes" hold down the alt-key until you can see the partitions.

If you were searching another way, ask again... maybe you could turn off the password protection via System Preferences>Security>More Options>Systemwide... but I can't test it right now, I am sorry