I think that "flagship" designation just emerged out of WP community discussions. Had Samsung released their Ativ S in more places, surely we would also have called that a flagship phone, no? The "Signature" designation was much more official, as it was used by both HTC and Microsoft when HTC announced their WP8 line up.
you can always force an update with Nokia phone with care suite tool, but if you have some other brands you are pretty much ****ed.