I got an update from the carrier that I didn't accept and it UN-rooted me. In the app, use ghost mode. Go out of the app, enter the code, and get the update. When you enter the code in the dial pad, Superuser3 is triggered from Play store. Its the only way to get 3.
Then it will update, at least on the z990 it did, and they are very similar.
Edit- SU Update fixer may help too. I dl'd it when I had that issue, along with the Super user Elite. I also checked out his web site.