We recently purchased a 20 year old (3) story home in NC.
The first floor has a 2 year old 12 SEER Carrier unit that needs no attention.

The 3rd floor (approx 750 of converted attic space w/ R-30 insulation) is being serviced with ducts tapped into the homes original 2nd floor gas HVAC unit (which resides on the 3rd floor). Several HVAC contractors have recommended one large system utilizing two thermostats (one for each floor) and the use of electronic dampeners. Others have recommended just adding a simple 1.5 ton heat pump and leaving the existing 2nd floor unit alone until it quits working. While a third option was to add the new heat pump and replace the older 2nd floor unit with a new unit as well.

Why is there such major disagreement among HVAC contractors on the use of the systems utilizing two thermostats and the use of electronic dampeners?? Is a 20 year old system worth taking a risk on??
Thanks