It will get you by.
If your ECM is giving you troubles and you've yet to source a new one, a quality used unit will get you by. If the price is cheap enough, you can use it as a diagnostics tool of sorts.
The vehicle's functionality hasn't been affected (it's the same part number) and the mileage is stored elsewhere and not in the ECM (thank goodness).
The issue would be that it's VIN locked, meaning that a proper scan tool will show the ECM's original VIN and not the vehicle it's now installed in.
Ideally, a new ECM should be sourced and coded with the VIN of the vehicle in which it's going to be installed in.
In the meantime, I recommend keeping the old ECM at hand in case issues arise when the vehicle undergoes VIN-related inspections (provided that its issues aren't detrimental to the vehicle's performance and occupant safety... like unexpected shut downs).
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