I just finished the permit process for this same challenge. Now I am deciding whether I really want to do the job. Since the rooms were only 11'x11' it wasn't a huge structural question but a question of insulation. The house is a 1 and 1/2 story so the peak of the roof is only 9'2" on the second level. It turns out R-34 is required if any additional ceiling space over 7' is added to the room. The solution to this problem is 2x8's sistered to the 2x4 rafters above the 7' level. The insulation needs to be polystyrene foam 7" thickfrom the 7' level up and 3" from the knee wall up so that no moisture is allowed into the space. The collar tie is still necessary within the upper 2/3 of the space so you really end up with an 8'ceiling. Better? Maybe. But it is still a lot of work to get an extra foot out of the ceiling. I think I may pass this off in the bedrooms but do the work in the landing at the top of the stairs.