2001 allroad; ECU was physically replaced through a fault and re-programed. Now I believe I fixed the original (old) ECU and want to try it. All Keys have been re-programed for the new ECU.
Would it do any harm to the car to plug in the old ECU and try? And would it perhaps work at all? Or could I encounter trouble with the immobilizer modules etc.?
Thanks in advance for your opinion and advise.

FWIW, keys aren't matched to the ECU, keys are matched to your Immo which is part of the instrument cluster. The ECU must also be matched to the Immo, but it likely already is.

-Uwe-