I had an RFI come in the other day. It was a request to change a couple dozen duct sizes on a building. The Owner, Mechanical Engineer and I were surprised. During the course of the design we ran clash detection. We felt good about the clearances for the ducts. The challenge with changing to many ducts is the impact on the system as a whole. But one step we always take is in our specifications is for the contractor to provide an above ceiling coordination drawing.