If you're not running encrypted drives I would not bother with changing from -p1 to -p2 as you will not reap any benefit.
Here is the message from the users manual page: FreeNAS® 8.3.1-RELEASE-p2 was released on April 10, 2013. This is a bug fix release which fixes issues with extending an encrypted pool and disk serial numbers not being displayed in the GUI.
Honestly the serial number was fixed in -p1, the disk encryption was fixed in -p2.