On my inside and outside corners; I only work with 1 foot sections. In other words on a inside corner, I have my compound cut in place and my corners made properly, but each side of the corner the crown molding is only 1 foot long.
Now, the method to my maddness is that I can then just add straight stock in the middles. Now in your case since your going from an 8 foot ceiling up to a 12 footer, you will have to angle cut the strait stock to make the incline and then another angle cut ont he other end to meet the straight stock. No big deal.
So, this way you do not have to adjust your corners for the increase in ceiling heights, you just cut your strait stock to attach to the corners.