See if the new unit has DIP switches to identify the location of the AC unit. Meaning if you have more than one AC unit, the DIP switch setting on the AC unit needs to match the thermostat zone id(i.e. zone 1 = front AC, 2 = mid, 3 = rear, etc.).
Also, if you have the old 5-button thermostat, and you install a new AC unit you may have to get the conversion circuit board so the new AC can talk to the old thermostat.