These 8' ceiling are closing in on me.

Anyone have experience opening up the ceiling and reshaping the rafters into a
cathedral or stepped-up open box (If that's the right name)?

I'm good with a saw and hammer, and can cut the hell out of boards, but I
know nothing about reshaping rafters and still have them strong enough to carry
the loads.

Is it as simple as adding boards to the rafters that will be in the final shape,
and then removing the sections that you don't need from the old design?

TIA