Vicki, who grows Heuchera for a living, says tidy them up round the outside in August. I reduced the size of one of mine quite a lot in August as it was so large and something had been chewing the outer leaves. Last autumn I dug one up from the border in a bad way and put several sprigs in a pot and they all took and bloomed well this year. I also repotted some into my big pots as part of a planting. They don't seem to mind being moved, therefore, and you can downsize them, but go easy this time of the year.