[jtrammel]

As long as the ductwork is installed and sized correctly for a zone system then a zone system will work fine. You have to find someone who really knows what they are doing though and zoning is inefficient. With 600sqft upstairs it's going to be a challenge to dump enough air for when only the upstairs zone is calling.

 

[jtrammel]

The best way to do zoning is with multistage ac/heat and have the smallest zone large enough to handle stage 1 of ac/heat then when 2 or more zones are calling stage up. Otherwise you either have to have a dump zone (wastes/overcools an area that used little) or a bypass which dumps excess supply air directly back into the return air (kills the efficiency of the unit and can cause nuisance problems such as freezing the ac coil, overheating the heat exchanger among many other things.)

Since your upstairs is so small it's probably worth zoning since any conventional system will be oversized since the smallest unit is a 1.5 ton.

Another option would be to use minisplit heat pumps upstairs and a standard system downstairs.

Any way you go it's going to be rather costly to do it right. Obviously the cheapest would be to keep what you have and reduct with zoning then later on down the road when that unit dies get a multistage unit.