The upgrade was delayed for nearly a month because of a broken Seagate EXOS HDD resulting in a service nightmare with Seagate (why I won't buy Seagate HDDs anymore you can find here )
Now everything is working again and today I decided to upgrade. Saved my config frist, then my encryption keys, my recovery keys and my HDD password, downloaded the update, reboot and... yes! Successfully upgraded from 11.3-U5 to 12.0-U1
Had to upgrade my ZFS pools and of course did the renaming from FreeNas to TrueNAS in my network settings...
Zero problems, so happy with TrueNAS! Thanks ixSystems and everybody being helpful!