I recently have uncovered that the previous owner had built an unsecured second story deck.  The ledger board was only nailed to the house in six spots and the 4X4 posts were set in dirt on a very small amount of concrete.  I have since lag bolted the ledger board to the house with the help of some friends and a pickup truck; however, I am not sure the best course of action regarding the posts and could use some advice. 

The deck is an "L" shape with a small 8'X10' section on one side and a larger 26'X15' section on the other.  Again the ledger board has been lag bolted to the house, decking sits on 2X10X14, with a double 2X10 beam running the 26' length at approx. 12' from the house leaving a 2' cantilever.  The beam again sits on 4X4 posts that are resting directly in the dirt on no real footers to speak of. 

First, It is my intention to replace these 4X4 with 6X6 for added stability, from what I have read from all of the other post, my joist span should be acceptable with just the one beam at 12 feet? 

Secondly, I have already placed screw jacks to hold the deck while I have begun to dig out the holes for footers and remove little concrete in place. However, I have read conflicting reports on setting the post 12-24" in the concrete and am looking for some guidance.  My plan (please correct me if it is wrong) is with the deck jacked up, remove the 4X4, dig down to the proper depth, tube and gravel, and place the new-notched 6X6 in the existing space, and concrete.  To me this seems a little less precise than working off a new footer pier and match and existing beam?!?! 

I appreciate your time and any input you might provide, this has been a nightmare!  Thanks in advance