OK. I actually trialled a system whereby a SIM can hold multiple IDs, and locks onto the local service (so US number in the States, UK number in England etc.)
WhatsApp just ran with the number I originally configured it on (which was a totally different t-mobile UK SIM number). Never changed it's settings to match the SIM ID I was currently using.
Once it threw up some sort of "you have 30 days to register" message and then just carried on working on the old phone #
I went back to my original SIM, so it was no biggie. But from the WhatsApp FAQs: WhatsApp ? Faq- change number