I'm assuming your ducting is running under your ceiling joists and increasing the height of your ceiling has nothing to do with the structural ceiling joist framing. The area of a 6" circle is 28.3 sq. inches. You can change over to a 3x10 duct with 30 sq. inches. It will allow the same amount of air flow HOWEVER, do not use any 90 degree elbows. It will reduce the flow significantly as well as create noise. Use a 45 degree transition. The radius turns that round ducting provide are better for air flow.