I noticed that mid-week on carib Princess they did change the bedspreads. I saw the stewards with piles of them. Our steward had 3 bed spreads and blankets both- stacked up on the closet shelf above where we hung our clothes, the first half of our cruise. So atleast on CP, the bedpsreads do get changed.
I would think since they were changed midweek-that they would also be changed whenever the cruise ended for the new cabin mates. These bedspreads were machine washable. As far as I am concerned-dry clean bedspreads should not be used on a cruiseship.