Seeing as how our ground really doesn't freeze in the winter, I think you could still do it and not worry about them being heaved out.

I continue to divide my irises well into late Fall as long as the ground is not frozen. I have had no problems.

Great! Thanks for the encouragement. I held off doing it, waiting for my re-bloomer (white, think it may be Immortality) to finish. Good to know it's still possible to divide the crowded beds.