Boom, new adventure: in place update from 16.04.2 to 16.10 to 17.04. As far as I can tell, nothing broke.
I also discovered that if the file manager manages the desktop, then windows don't remember where they last closed; but where, I guess, GNOME itself manages the desktop, blammo, windows and apps open where they were last closed... mostly.
Next test is the acid test.... LibreOffice. And that's for tomorrow.