Can you post an image of what you are trying to do and explain a little further? Most ducts I've seen drawn change shape from circle to rectangle or other shapes and typically are done using the loft or sweep commands. But showing me an image and a little more detail can let me give you a more definitive answer.
If you are using sheet metal, I would suggest using the "Lofted Flange" tool inside of the Inventor Sheet Metal tools. However, if you are modeling for other reasons, for instance 3D space planning, I would create a 2D sketch of the cross sectional profile for the duct work and then a 3D sketch for the duct path. Then you could use the "Sweep" command to create the required duct work. If you have any questions, please do not hesitate to contact me. Hope all is well and have a most blessed night!