Okay, it went rather well. Making the .ova and disks files work together just needed a bit of editing of the VirtualBox.xml file and fiddling with the Virtual Media Manager, and then all was ok.
I then followed the steps needed for an upgrade to v.13, and I seem to have succeeded.
Whonixcheck displays all green lights and sanity checks results are fine, but how can I be sure that I really succeeded ?
-Whonix packages dependencies are version 3.4.2-1, that’s good?
-One thing I’m not sure about: the repository for updates is the Jessie repository, should it be that one ?
-Is there something else I need to run or test to verify that I’m all good ?
(Actually, I was like sure that it was going to fail, after one year and a half without having touched Whonix. And now, the simplicity and smoothness of how it went is just making me doubtful that all is really in order.)