but, are you going to remove the roll shingles? How many layers of shingles do you have on the area? Usually a roof can handle two layers or at least where I live its acceptable.
If you use the existing roll in place, do cover it with tar paper again. Then lay your comp shingles down and on each course, overlap the throats more than usual since you do have a 2 in 12. Use Butyl Rubber Caulk around the light area.