For Revit ducts you can tab-select a run of ductwork and then on the ribbon a “change type” button should appear. Click it and in the properties pallette, click the type drop down at the top and select your round duct.
It should convert although you might get some disconnection errors if fittings are too close together to re-make the connections.
I can’t advise on fabrication ducts because I don’t use them.
