There is no such process to roll-back to the prior OS version and downgrade as well all the applications and their settings to the supported in Expedition.
The only thing I could advise is to do a database backup (look for Mysql/MariaDB databases backup), and make a copy of the /home/userSpace folder.
Then, create a new instance of Expedition again and restore the /home/userSpace folder and recover the MariaDB databases.
This may get you back and running to an Expedition running on Ubuntu 16.04.
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