You'r correct about the engine change but DVLA will need to be told and the V5 up-dated.
Also the correct details will have to be confirmed with an invoice or "statement" in writing from a reputable garage.
With regards to the BMW V12 if I remember correctly this engine uses 3 ECU's. One for each bank of cylinders and one master, which is then linked to the gearbox ECU, that's a lot of wires! !