The big thing to remember when going up in sizes on a dually, is that you need to make sure the rear tires DO NOT TOUCH when loaded. If they touch when the truck is loaded, you will wear out the sidewalls of the rears, which will lead to a blowout that will cause a LOT of damage.
There are a couple ways around this, you can get new rims with different offsets for the rears, to space the tires further apart, or you can run wheel spacers to spread the tires out. I don't know about rim weight ratings, but if you get too big of a spacer then you WILL reduce your cargo carrying capacity - by how much depends on the spacer thickness, and offhand I don't know what the reduction is.
If the truck isn't going to be loaded very heavy (or is just a pavement queen), then you'll be fine doing either of these. If you're going to be routinely loading the truck heavy, however, you'll want to be careful with what you are going to do.
