Anonymous wrote:OP here. Would it be ok to just turn off the upstairs thermostat?
It's your house, you can do whatever you want.
If the zoning isn't working properly turning off the thermostat isn't going to fix it.
A thermostat is an on-off switch. When the temperature is above the set point it's off, when the temperature is below the set point the thermostat is on. If the temperature upstairs is already above the thermostat set point, the thermostat is already off. Turning it off doesn't make it "more" off.
If the upstairs thermostat is off -- either because you turned it off, or because the temperature is above the set point -- and there is still hot air coming out of the upstairs vents, your system isn't working properly.