Bearded irises can pretty much be transplanted any time the soil is not frozen. The reason most people plant them in late summer is so that they don't disrupt the bloom time in the early summer. Where I live, you take a chance of having them heave out of the ground if planted too late in the fall, as the freeze thaw process will push them out of the ground if they haven't had enough time to establish a good root system. You can still plant them, but you'd be best to put a brick or somethng heavy on the rhizome to keep it in the ground. Most vendors of Siberian irises send them in the fall for planting. I have transplanted some in the summer when I just dig them from one spot and move them to another and it seemed to be OK in my climate. Leslie made it clear that there is often no one right answer to that question, and conditions in ones area may require different solutions.
