Ideal time to do it is in spring just as they are coming through and you can see where the growing points of each part in the clump. Second best is early autumn so they can get their roots down a bit before the tops die off in winter. Just take a sharp spade and go for it. As long as you have a good bit of root which each part there should be no problems.

And don't do it during a drought! The soil needs to be nice and damp before you lift the plants.

you can do it in the early autumn too, replant asap and they have time to settle in before the winter.