While I would recommend starting your Booksonic Air fresh I understand that in some cases migrating users might be needed. Just keep in mind that there is a risk this will lead to weird bugs.
1. Stop your Booksonic legacy server
2. Go to C: and rename the Booksonic folder booksonic.old
3. Run the installation described in this blogpost, then open powershell as admin and run the command stop-process -name java
This will shut down you booksonic air server, please keep in mind that it will also shut down any other java process you may have running.
4. Go to c:\booksonic and delete the file airsonic.properties and the folder db
5. Go to c:\booksonic.old and copy the file booksonic.properties and the folder db to c:\booksonic
6. Rename the file booksonic.properties to airsonic.properties, do the same with all the files in the db folder
Now you can restart the Air server by rightclicking on start-booksonic.ps1 and selecting Run with powershell
You should now have your old settings and users, if you run in to any problems, you can contact me at firstname.lastname@example.org