In Autumn 2009 I found an ailing Heuchera (Caramel) in my border, overwhelmed by other plants. The middle of it was rotten, but the rest of the plant looked healthy. I filled a big pot with fresh compost and snipped those shoots near the middle and planted them. They have all survived outside. Some have been used in other plantings. Have a look at Vicki's blog. she says not to disturb the central part of the plant. I would say, if you want to down size a large Heuchera, take some side shoots now ,and see how it goes.

In general, you should be able to dig up and pull an old or existing plant apart, pieces with some root attached can be separated quite readily, remove most of the older leaves leaving a few younger shoots, can either be planted into pots or straight back into the ground...anytime during the growing season seems to be fine.