The easiest and fastest way is to boot into recovery, either TWRP of CWM, and do a factory reset, then clear cache , then clear Dalvik cache. After that has been done, then all you need to do is flash CM 9 and the Gapps and then reboot. I have done this many times and it works fine. The only problem is that you close all of your apps and data. But as soon as you go to Google Play all of your apps will download automatically. IF you created a Nano backup....then just do the factory reset, clear caches then restore your backup. This was is WAY easier and you don't lose any or your apps.