Converting your old roof to a cathedral usually involves extensive restructuring of the roof loads and how they are carried. This is usually "do-able". You could be faced with having to add a structural ridge beams with supporting posts all the way down to bearing, which could also require new footings, etc. Usually, if you don't do this, then when you remove your ceiling joists/collar ties then the top of your walls want to spread and the roof falls in. We can't see how your house is constructed from here and I wouldn't even trust most architects but I would insist on structural engineer drawings in addition to architect. Good luck.