The Match Ducts command is used to copy the properties of an existing duct on the drawing to one or more other ducts. The Starting Elevation and Ending Elevation will not be changed.
To match duct properties, go to
Ribbon: DM HVAC->Ducts-> 
Match Ducts
Pulldown Menu: DM HVAC->Duct Centerlines->Match Ducts
You will be prompted to select a duct from which the properties will be copied.
Select duct or label to copy properties from:
You will then be prompted to select one or more ducts to change.
Select ducts to copy properties to:
Press ENTER to change the properties of the selected ducts.