Don't know the answer to your question, but out of interest is this a legal requirement or something you want to do?
I bought a bike from NI last year and it's registered here now with the original plate.
Absolutely no bother to change,ring Swansea and they will run you through what to do , Have done it twice as bought two bikes in England. Got new reg and V5 in a week.
I brought a WC back from Nottingham last year - reg had RUC in it. Didn't half feel self-conscious riding it up from Dublin. Changed the reg immediately!
I've my 3rd GS brought in from England, once when I was changing the address on the V5 I was asked did I want a local registration, I never have bothered. Is it still free to change over though?