Though the version jump from 2.9 to 3.1 may not seem like to much of a jump, it did not go easy for me. It failed the first few times. I tried following the directions at dotNetBlogEngine.net, but that was unsuccessful… all my existing posts would not show.
I finally found a comment to a post on CodePlex for problems migrating from 2.7 to 3.0 where user MGD_King shared what worked for him.
Sep 22 at 9:34 AM
I had a similar issue when I upgraded. What I found was I had to delete all of the previous contents of my App_Data folder and use the stock files in order to get the Admin panel to work. Then I manually copied over my Posts, Pages, and Files directories and then manually set up all of my widgets.
This new approach worked for me and I wanted to share it so maybe you found this solution a bit easier to find.