The lower level only has 3 vents so you must have a bypass damper.when upper level is off 2000 cfm will try to flow out lower level, extremly loud and noisy.a large bypass will slow the air down.I mostly install 2 main ducts with it own zone dampers.I would go that route. And i would question the guy who suggested no bypass and wait any see.

I wouls be skeptical of any of these proposals.
There are 2 problems with 2 zoning from one zone, namely
If only one is calling, then the airflow to that zone would be too high and cause excessive noise which necessitates bypass ducting which the bidders correctly mentioned
What they did not mention was that the furnace was designed for total load and as a consequence you will encounter higher than normal temperatures across the heat exchanger when only one zone is calling, causing high temperature furnace cutout and possibly premature failure of the heat exchanger.
Cooling by this method would cause its own set of problems.
If your reason for 2 zones is that there is an imbalance then I would not do it, but instead install individual dampers to balance the loads. Otherwise, if you want to totally shut down one of the zones at times then the only way you can reasonably do it without major ductwork changes is to change the blower to variable with all the other features suggested.But this is an additional cost to an already costly problem.
Bottom line Furnaces and A/C systems are poor candidates for changing to 2 zones. Sometimes it is even more cost effective to install another furnace and underfire the existing one to take care of the two zones.