You don't specify if you're doing work for display (RGB) or for print (CMYK). That green, with its 100% saturation, would probably cause you problems when printing.
Illustrator is trying to help you. It's trying to keep you from using colors that cannot properly display or print. This is what color management does. The color that you are trying to select is outside the gamut of the color model that your CS6 applications all are now set to use. This was likely changed when you upgraded.
In the color picker screen, there are some controls that you probably never paid any attention to, which I have labeled in the attached image:
- The little warning symbol to the left of the OK button appears when the color you're specifying is out of gamut.
- The box showing what Illustrator suggests converting the color to, to stay within the color model.
- A little box icon, which I think is supposed to represent a color model or something.
- Another box, which on my CS5 shows the color that is specified in the picker, not the corrected one. Not sure why yours shows the corrected color; maybe CS6 is just EVEN MORE STRONGLY suggesting that you correct the color.
In order to get Illustrator to accept the out-of-gamut color, click 3 or 4 (at least on CS5 I can click 4). You could possibly have an issue with that color displaying consistently across different displays. Since I don't know what you're designing, I can't say how good of an idea that might be.
Or, you can change your color profile. In Illustrator, it's under Edit > Color Settings. You'll probably need to read up on the arcana of color management to determine what is best for you.