Is it a good idea to "upgrade" my SIM

I have an older version of the Global SIM card which came with a 0044 IOM number only (I have never activated the US number) I see that current Global SIMs come with a 02 mobile number which seems to be reachable from the UK by dialing just the phone number (i.e without needing to dial 0044 or +44 before it) Is this correct please and, if,so, is it possible to upgrade or do I need to start over by purchasing a new SIM? If I do need to buy a new SIM could my call credit be transferred to the new SIM?