Why bother retaining the NI plate if you replace it? It's probably worthless once you've paid the fees.
I've bought two cars from NI now, one of which was new and on NI plates. I've put personalised plates on it, which I will retain when I sell the car. At that point I don't know whether the original NI plate will be reissued or if it will be given a more specific geographical and age related plate. If the car had originally been registered in England, it would almost certainly be reissued with its original plate.