i think its a lose lose situation if you cut back two rooms your making that part of the house colder whjch is going tomake the surrounding area colder which is going to spread to the stat and make the system run the same amount if not more. just close yhe dors and leave it alone. if you zone your just bypassing the air you already heated to send to them rooms and sending it in a big circle and the unit is stil going to run until it satisfies te stat if your going to heat it why not use it.
I would do energy audit, blower door test, just to see if the house envelop can be improved.
Check to see if the ductwork is leaking.
May be wrap flexyductwork in the attic.
Insulation could save you more than some of hvac upgrades.