I have done it before with no ill effects. Some TBs are going to take a year to re-establish no matter when they're divided. The rhizome that is producing a flower stalk this year is the mother. She will not bloom again but she will give her energy to producing offspring. Don't toss her. The dried up rhizomes (still attached or not) are the ones to toss. Leaves are usually cut back by 1/2 when replanting so the plant stays upright. Give a deep soak, then no more water for a month.

Molly Price in her classic book on iris recommends dividing immediately after bloom. It has become popular to wait until late summer but I have not seen documentation that this is preferable.