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do you have more than one thermostat and you have a zone control panel that the thermostats wire into?
This control panel needs specific thermostats. That is why the numbers/letters differ.
Instead of just being able to change a thermostat to a furnace/ac...you are actually changing a thermostat to a zone panel....which is not compatible with your thermostat.
You would need to change your zone panel or convert to a 1 zone system