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You need to remember that this is wired up as though it would be the only A/C up there, so you won't need much. Most of these are wired up so that the second unit just needs the 120 VAC and the RJ-11 cable that connects the two units together. I don't think you have to change out the board, but if you give me the actual model numbers off both units I can check just to be sure. All follow ups are included in our fee.
Can you attach a photo of that for me so I can see what cable you have?
One is the freeze sensor and plugs into that spot and the other should be the communications cord. Did the new unit come with an installation guide? The wiring should be in there.
Just be sure you pay attention to the plugs. They are backward from a phone cord. Look at the wire colors and positions so you can be sure they are correct. Let me know if you need more later.
Just checking back in to see if you got all you needed on this question from the information I supplied or do you still need more input here?