They tend to give upgrades in the same bed type of room, unless you note that you'd prefer a king if it becomes available. I'd email the property and ask them to note that in your reservation. I'd never expect an upgrade as a discoverist, though.

Note that if you have 2 people staying, their default seems to be to push the two twin beds together. This ends up making what appears to be a giant mega-king bed at first glance. There is a slight divot in the middle, of course, but the beds are so firm you don't really notice it, and they have sheets/covers which span the entire double-twin. The twin room is also slightly larger I believe, so in general the twin is considered a superior room. Unless of course you don't want a bed that is so massive-looking.