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Apple made some changes in 10.2, making it a lot more difficult/impossible to change certain icons...

The file you want is /Application/System Preferences.app/Contents/Resources/PrefApp.icns

It has the three different sizes for the icon embedded within it.
 
No, you just need to use an icon creation app to do it, and then save as a .icns file...or if you already have the .icns file from another app, use that.
 
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