This isn't really a question about the HVAC itself but the house. I'm looking at this older home to purchase and all the rooms in the house have a low ceiling except the front two rooms. When looking in the attic space on the 2nd story you can see there is like a 3ft gap between the first and 2nd floor. The HVAC is ran between the floors like that and I honestly have no idea how that works if I want to raise the ceiling. Anyone else ever see something like that.
My 1st fl is about 9' and second is 7.5. Adding next to it, if the 1st fl ceiling and 2nd fl floor are same joist, I need 1.5' wide joist-which is engineered, expensive and impractical. Sorry, not applicable to you but writing it out helped me organize that part of my house. I meant to say such space is seen all the time and yours is not that unusual.
Horizontal duct can be moved up to a new hole in the vertical trunk and old hole closed. Splitting a duct is not that simple, however, and wrong placement can ruin the balance that's there. You should draw a diagram and ask in the hvac section.