Also wanted to add, hugely thrilled to see that Whonix 12 -> 13 upgrade will be possible in place if desired, without restarting from scratch and a fresh installation. For some that latter option may be fine, but for someone like me who installs A LOT of software on Workstation, the prospect of starting over and redoing it every time, is horrible.
Is that update method only possible this time because Debian 8 Jessie is still the base version for Whonix 13? Meaning when a Whonix version based on Debian 9 Stretch is released, we will still at that point have to do a totally fresh install? Or will an in-place upgrade path always try to be provided?