Rollover for the manual
Use phpmyadmin to backup all of the wiki tables!
Locate all pages on the wiki that have 3.0 in the title - from the wiki: toolbox, Special pages, All pages to get a full list of wiki pages. Copy/paste it into Open Office word processor - it is in three columns separated by tabs. Use regular expression search/replace to replace tabs with carriage returns - now have a list of all files on the wiki, one title per line. Save it as a text file (I'll call it myfile.txt for this example).
Use grep to find only the pages with 3.0 in the title: grep "3.0" myfile.txt > mynewfile.txt
Use a text editor to view the new file, and delete any pages that you don't want to roll over (That is, some pages may refer to 3.0 in the title, but are not pages that we want duplicated into 3.1...)
Copy/paste the new list of page titles into the wiki export using toolbox, Special pages, Export pages and create the xml to screen. Be sure box for Include only the current revision, not the full history is ticked.
Copy/paste the xml into a text processor and use search/replace to replace 3.0 with 3.1
Save the file (I'll call it export.xml for this example).
Import the new pages into the wiki: toolbox, Special pages, Restricted special pages (available only to Sysop members of the wiki), Import pages. Browse to find the .xml file and Upload file.
There are serious restrictions for both file size and processing time for the scripts - I had to split the .xml file into 8 pieces to get it all uploaded...
That should now have created all of the new pages.
New pages created by this sort of import do not automatically get added into the search index of the wiki. Use phpmyadmin from the cpanel and use the repair tool for the searchindex table.