well, if you are "dead set" on it 
;D :D,
then the thing to do is find a changeover switch and see how much physical force it will take to move it. I'm guessing it's a lot. Design-wise though I would guess that the Arduino stuff will cost you under $150 and the changeover switch will be $1000-2000, so it might be more cost effective and less life-threatening to just buy an automated changeover switch. Something to think about.
Oh-- don't forget to get some rubber boots. ;D