So no more Icedove? Mine went ahead and updated and migrated my Icedove settings. (Once I had done an 'UP', the next restart of icedove resulted in the migration) Sure enough, it says 'Thunderbird' in the window title...
Apparently -
Icedove 1 (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=816679)
icedove 2 (https://lists.debian.org/debian-devel-announce/2017/02/msg00004.html)