Best time is immediately after flowering, generally late October or November through till the end of January. They can be moved later but next season’s flowering can be disrupted the later you plant them. If you have to move your Iris at another time, it is okay to do this but depending on how late it is, they may not flower until the following year. We have irises flower being replanted as late as March to early April but any time after this, they are unlikely to flower the next Spring but should be fine the following year.