sorted his ECU vin mismatch maybe he can help.
Nope! I ended up living with the donor VIN. However, you can reconfigure most of the rest with SDD. (Dealer Options).
One of your problems may be that the gearbox calibrates to the more powerful 4.2 and then throws errors. Not sure if it also messes up engine mapping and fuel pressures, etc.
Did you compare the donor ECM Jaguar part number with the original ECM? If identical the rest is all programmable except the VIN. Getting this changed by Jaguar potentially leads to Police interviews as they are obliged to report apparent "ringers". Could at best mean lengthy explanations and proof of ownership (keep hold of your original ECM), or worst being taken off the road.
If you enter a Diagnostic session in SDD, select communication as the only symptom then you should be offered an Update ECM procedure. I believe that you can enter an alternative VIN and engine, but I have never run the procedure or put it to the test. There is a TSB on the subject somewhere.
If you can get that sorted then there isn't a need to shop for a 3.5 ECM.