is it Ok to cover an existing sheetrock ceiling with 1/4" sheetrock? I am concerned about sagging with 1/4" and I was thinkin about using 1/2 or 5/8

we are doing a bathroom remodel (down to the studs), but we left the existing sheetrock celing up, since the ceilings are high and we know we wouldn't miss a half inch in height. The old ceiling has been dammaged a little by the removal of a closet, and it would be difficult to patch due to the "swirl" texture on the ceiling (actually the only real dammage is that the swirl is missing where the closet framing was attached to the ceiling)

also, should I sand or skim coat the swirl before putting up the new sheetrock?

I am not using greenboard on the ceiling because of sag issues (greenboard is no longer being reccomended for ceilings, even in bathrooms). The existing sheetrock is almost 40 years old with no water dammage or mold