I waited a while before taking the plunge for the same reason. If you have not already done so I would recommend that you download the latest version of Avisio from theme forest and study the provided document which lists the files changed. There are not that many to replace. My big concern was style.css but the only change in that file was the version number which you can just correct by editing in your current file. The others files, in my case, did not effect the changes I had made so all I had to do was replace those. This is my site http://www.ourgolftour.co.uk which you can see has had quite a lot of tweaks.
Do download the new version and take a look at what has to be updated there is not that much to change. Obviously you can check this all out before committing to an updating to 3.1
It all depend on the changes you have made and only you can decide.