I have a renovated townhouse in a 125yr old building, with insulation and thermo pane windows, First floor; 3 rooms, open floor plan. Second floor; 3 bedrooms & bath. For numerous reasons, location of a new a gas furnace/ AC system in the attic was very attractive. HVAC contractors said it would work OK. All supplies are in the ceiling near outside walls with AC type diffusers. A single return is located on the second floor in the hall ceiling near the stairs. As might be expected the AC works fine, but heating is a problem, bedrooms are hot, unless there dampers are almost closed (not good for AC). Under this set up for heating, the floors are fairly balanced and uniform if one temp is held, but if a temp set back is used, when returning to higher temp, there is great imbalance between floors and there is also stratification especially on the first floor.
Supplies in the ceiling must stay as is, but diffuser style is an option.
It is an option is to add a return on the first floor neat the center of the unit with a volume equal to the first floor supplies. (I think this is a must do) This return would be easiest located near the ceiling, but with much more work, could be located near the floor, is the lower location better and worth the extra work?
Is the second floor hall ceiling return OK (being reduced in volume by the amount added first floor return) or is it worth much more additional work to add returns in each bedroom, and if so should they be high or low?
Are the current ceiling diffusers OK (spreads air parallel to ceiling) or would a more down directional diffuser be better?
What is the best compromise here for a combined heating and AC duct system?

More estimates and more ideas taken from the plans is always a good idea.