In order to do this, you will need to manually edit the Settings.sav file using a hex editor. Here's how:

1. Download the stand-alone/portable hex editor HxDen (I used version 1.7.7.0) from http://mh-nexus.de/en/downloads.php?product=HxD or http://download.cnet.com/HxD-Hex-Editor/3000-2352-10891068.html?part=dl-HxDHexEdi&subj=uo&tag=button

2. Open up the Settings.sav file (located in \SteamApps\common\Magicka\SaveData\) in HxD

3. Find the second space after the QWER ASDF line. That is the character that corresponds to Shift/Arena Cast. Replace that 'no-break space' with an 'inverted exclamation mark' like this one: ¡

4. Save the file and then close out of HxD

5. DONE! :D

Here's some screenshots of how the Settings.sav file looked like in HxD before and after I changed all of my keys to the right side of the keyboard:

http://i.imgur.com/XatXhib.png
http://i.imgur.com/8W7XmAH.png



P.S. The way I figured this out was by changing the Shift/Arena Cast key from Left Shift to Right Ctrl and then finally to Left Ctrl, and just looking at how the Settings.sav file changed. This table was what helped me figure out what the value corresponding to Right Ctrl was:
http://www.alanwood.net/demos/ansi.html

Here's the values for some commonly used keys in Magicka, in case you were wanting to manually edit some more stuff:
= left shift
¡ = right shift
¢ = left control
£ = right control