Hey @breugelmans ,
to do the upgrade in the safest possible way, first make a backup of your database and your Fidus Writer directory. Then upgrade from 3.3.2 to 3.3.13. When that is done, upgrade to Fidus Writer 3.4.9 (lats 3.4 release). It is important that all your documents have been updated to the correct version at this stage. To upgrade further, you should exit your python 2 virtual environment one and create a new virtual environment for python 3 following the install instructions for python 3. The database and media files stored on your server will be the same as before, so the end users will still see their document. Make sure that you can run the server as before using the Python 3 virtual environment. Once you are certain that it will work, you can remove the Python 2 virtual environment folder and do an upgrade to the 3.5 version.