Before dist-upgrade my default session was GNOME.
After dist-upgrade my default session is XFCE.
The KDE, XFCE and GNOME sessions were installed before the dist-upgrade, and are still installed afterwards.
The default session should not have changed.
I can't remember if I had manually set the x-session-manager alternative before upgrading, it isn't set to manual now.
Before dist-upgrade my default session was GNOME.
After dist-upgrade my default session is XFCE.
The KDE, XFCE and GNOME sessions were installed before the dist-upgrade, and are still installed afterwards.
The default session should not have changed.
I can't remember if I had manually set the x-session-manager alternative before upgrading, it isn't set to manual now.