Raising second floor ceiling

<p>Hello</p>
<p>My second floor has 8 feet ceilings, except for the bonus room that has a vaulted ceiling.</p>
<p>I want to increase the height of ceiling in 3 bedrooms and a bath.</p>
<p>All bedrooms and a bath are on the outside perimeter of the house, meaning each has an externally facing window.</p>
<p>In the attic the structure that holding a roof is a truss with collar ties on every rafter and rafter ties/ceiling joists.</p>
<p>The roof is complex type, so bonus room has its own rafters.</p>
<p>Currently there is no working HVAC in the attic as the one that is there has been broke for a while.</p>
<p>The original ductwork is still in place and ideally I would like to leave it where it is.</p>
<p>The A/C vents are on a ceiling in all upstairs bedrooms.</p>
<p>Ceiling does not have to be raised uniformally in each corner of a bedroom.</p>
<p>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.</p>
<p>How can this be done safely to make sure that the external weight bearing walls will not cave out or in?</p>
<p>In other words, how can this be done to ensure the roof with its load will be properly supported?</p>
<p>Thank you.</p>