If you are going to do it this year do it now.

You can do it right now as aquawise says or next year after bloom is over.

I've been replanting Iris since late July. The earliest ones are growing nicely and the later ones are peaking out of the ground. I'm expecting nice blooms next spring. Don't think it's too late to replant now because iris like/need freezing temps to grow.

Jas, not all irises like freezing temps, only certain ones. Tall bearded irises grow exceptionally well in California and Arizona without any freeze at all. Dwarf irises do not bloom without a cold spell, though.
Renee