Can I reduce the floor register boot size in my old house?
Many of the very old houses had gravity style furnaces that utilized large registers and grills that offered very low resistance to the flow of air. Since there were no fans or blowers the delivery pipes and registers had to be very large. Depending on the size of your room/house update it may be a good idea to ask an HVAC company for a bid on a duct work and register replacement package. You do not want to restrict the air flow to the furnace today and have to do it "again" if the work is done wrong. my 2 cents