Unfortunalty the BeCM is not plug and play....it needs syncing tothe other ECUs...also it could alter the Odometer reading idf the donor BeCM is from a higher mileage vehicle.
Also there are amny settings that need to be transferred from your BeCM and the re-written to the donor BeCM....
I am not saying it is impossible, but you would need a full fat diagnostic device to set it up afterwards....
Not to mention you would also need the wiring loom, the Seat ECU(?) and the paitience of a Saint to sort out the niggles....
The P38 is a complex beast - there would be little point in making it more so, by introducing new issues....
It can be done - just need to realise the headaches it could cause.