What I've previously done is drag and drop all the songs to a Service and save the service (to an .osz file). Then in the new version of OpenLP I just open up the saved Service file. In settings I make sure that "Settings > Songs > Import missing songs from Service files" is ticked. Then when you close OpenLP, all the songs will be added to the new database.
You can also export and reimport songs, but I found that this would lose the verse and chorus repeat orders.
Both methods avoid having to do a database upgrade.