A new converter tool is now available which allows you to convert current SQLite worlds to MySQL. In addition to that, you can also convert MySQL worlds to SQLite. Download: https://download.rising-world.net/tools/converter.zip
We highly recommend to create a backup of your world before converting it 
The converter provides a graphical interface where you can set up the path to the world folder and the login details for MySQL. If you want to use the GUI, just start the program via double-click (alternatively you can use the command java -jar converter.jar).
Alternatively you can run the program from the command line, but then you have to provide the login details as arguments. Ideally you can run the program with the argument ?, then the required parameters are displayed and explained.
In order to run the program, Java 8 needs to be installed.
Note: If you convert an SQLite world to MySQL, it's necessary to keep the old world folder since it's still used to store uploaded user images as well as some other world information.
