Number Plate Transfer Question

[MBZ 6]

I have tried to ask the DVLA this question all day I must have rung 10 times and ever time I get a message telling me that all lines are busy and to ring back later

We have a car which is SORNed until April( soft top for the summer)with the plate J1 DYD.
I want to put J1 DYD on retention and put M600 onto the car (I have M600 on retention)
The Question is can I swap the plates while the car is SORNed. I want to swap the plates in March by post and tax it in April.