Vonda, I think, the 3.1 upgrade is not a program upgrade but a patch to correct some bad problems in version 3.0.
I do recall that 3.1 upgrade (or patch?) cannot be applied to any version except version 3. You can't use it on 2.8 or 2.9 or 2.5 or. . . or. . . or.
So, if you have already installed 3.0, go ahead and add the 3.1. If you stopped at 2.8, which was a pretty good version, consider just stoppin there. The 3.1 only corrected a few things that only appeared in 3.0 but did not appear in earlier versions.
Of course, 3.0 was also the only version that allowed global editing. A small price to pay for not having those 3.0 data corrupting bugs.