Hello

My second floor has 8 feet ceilings, except for the bonus room that has a vaulted ceiling.

I want to increase the height of ceiling in 3 bedrooms and a bath.

All bedrooms and a bath are on the outside perimeter of the house, meaning each has an externally facing window.

In the attic the structure that holding a roof is a truss with collar ties on every rafter and rafter ties/ceiling joists.

The roof is complex type, so bonus room has its own rafters.

Currently there is no working HVAC in the attic as the one that is there has been broke for a while.

The original ductwork is still in place and ideally I would like to leave it where it is.

The A/C vents are on a ceiling in all upstairs bedrooms.

Ceiling does not have to be raised uniformally in each corner of a bedroom.

Instead, I want to follow roof's design, so the ceiling by the eaves will be as is but higher toward the internal central point of the structure.

How can this be done safely to make sure that the external weight bearing walls will not cave out or in?

In other words, how can this be done to ensure the roof with its load will be properly supported?

Thank you.