Cutting back early is fine, and won't hurt the tubers at all. The tubers will continue to mature under the ground, and will send up new shoots right before the killing frost. Having those new shoots frosted is no reason for concern, either.

Serious growers in moderate climates dig 'early' all the time to miss the fall/winter rains that can rot out their tubers. As long as the plants have had ninty days of tuber maturing time, they should be hardened enough to make it successfully out of ground... As you don't dig, there are generations of mature tubers underground, so that's not even an issue. Overcrowding might be, though, which is why it is generally recommended to dig and divide clumps at least every other year. This invigorates the tubers, and I'm told, improves the quality of blooms.