I am afraid you have to.
My guess is that you wish to retain the option of working with older documents without their being caught in the different incompatibilities.
As you already know, they have reversed the Y axis in CS5. But you can actually unreverse it and get back to what you were used to in the older versions.
To change the ruler origin (and direction of the Y axis) in CS6 (&CS5 (&CC)) to be the same as in CS4 and earlier, you may:
A) Install and run the script in the OP of this thread,http://forums.adobe.com/thread/890507?tstart=30
B) Manually change the relevant preference settings as follows:
0) Close down Illy (you can only change the preference file with Illy closed);
1) Find and open the AIPrefs (Win speak) or Adobe Illustrator Prefs (Mac speak) file withe a text editor like Notepad or TextEdit; the preference file is a (hidden) file in the (hidden) folder Adobe Illustrator CS5 Settings
2): Find and change the following two bits of code:
/isRulerOriginTopLeft 1 >>> /isRulerOriginTopLeft 0 (change 1 to 0)
/isRulerIn4thQuad 1 >>> /isRulerIn4thQuad 0 (change 1 to 0)
This is a global change so you only have to do it once (just as you can undo it once).
You may find the folder as described in <em><strong><a href="http://www.bugge.com/Family-and-friends/Illy/illy.html"><span style="text-decoration: underline;">Move the folder</span></a></strong></em>.
Don't think I can just install CS6 and be done?
If CS6 is all you need: yes, that should be possible.
You might need to enter an original serial number somewhere along the way, but you shouldn't have to install all of these versions.
In case you need to be downwards compatible with the gaussian blur or drop shadow/blur/glow effects, then you would need to install at least version CS5 additionally