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Hi I'm Brian and I'm here to help. I'm sorry your question slipped through the cracks, I saw it on the board now for the first time.
What is the model number of the new thermostat you want to use? Only certain thermostats interface with the EIM panel (THM5421C). If your new thermostat is not one of them, you will have to bypass the EIM and wire the new thermostat directly to the Smart Board. If you do that and have anything wired to the U relays on the EIM, you will lose that functionality.