I keep my asters pinched back to 4 inches at first then to 6 inches and I keep piching until the second week of july rather than the 4th of July. Transplanting now may be fine if you take care to take a large dirt ball with the root mass disturbing the roots as little as possible so the plant won't have to work as hard to repair damaged roots. You can also help ease the transplant by preping the hole by loosening the area well around the transplant making it easier for roots to expand into the surounding area. once the plant has died back mulch it well for the winter.

I don't think you are supposed to transplant perennial mums in the fall - you just might kill the whole lot. The root mass will come up very easily in the spring and even the smallest divisions will flower by Autumn.

You might consider pruning your asters more than two times -by 75% in early June, and again in late June, and again in mid July, cutting them way back every time. The ones I did this with this summer eventually reached about three feet tall and are very bushy.