I was just browsing the forums and saw your post and wondered what your outcome was.
I haven't got an answer.
I did have FC3 but uninstalled DCS in 2022, then reinstalled it last month because of new AC in FC4.
I can't see a reason not to upgrade except it has a small cost. Also it seems there may be some content changes, eg there is an F15 mission, that I really enjoyed, that is not listed anymore.
So I don't know if you must upgrade to FC4 to keep using flaming cliffs but I'm curious if you were able to upgrade successfully m.