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Hello and thank you for your question,What you have heard is correct, replacing it with a used one even after programming may not work do to a conflict with the existing software. You are best served to replace the ECM with a factory re-manufactured unit or even a new one.
I Have tried doing this and had poor results which is true in most cases but if you would like a second opinion I can open it back up.
Yes a new unit will require dealer programming for sure. Ill open the post back up for you .