I have heard that it is possible to swap the becm for an other unit, but you have to change the locks and keyfobs as well.
In my opinion you could just change the keyblades from your car with the one in the fobs from the old car.
Installing the new BECM will give you a fault regarding miles but this will automatically be resolved by your becm updating its milage to the highest registered from the donor car or your own car. So in case your donor car has a higher milage the milage of your car will jump to that value.
After installation you have to synchronise the engine ecu with the new becm and your car will be ok to drive again. The vin in the becm will be that of the donor car but that by itself is not a problem. Using the ECU from the donor car might save you this last step.
The above replacement procedure has been succesfully performed by an independen garage here in KL. They import cut up cars from Japan and us the components to keep the cars here on the road. Import duties on new cars are as high as 300% and for new range rovers the lowest price for a new TDV8 is close to US$ 200K.