Tammy, many times a GTY is less expensive than booking an assigned room. DCL will offer GTY rooms in order to fill the ship. There really is no advantage other than some $$$ savings or that being the only way you can book the catagory you want. (the cabins are all selected)
Since they are less than 60 days out, yes they can cancel and rebook. The usual problem is that the cruise has probably gone up significantly and you would pay the going rate today not what was paid during the initial booking.