on 23-07-2012 21:56
You keep the same sim card and changes normally come into effect on your next billing date. Check on My O2.
on 24-07-2012 16:09
You keep the same number, same sim card.
The changes will take effect within 24 hours of the upgrade being processed, so your next bill will be pro-rated (credit back for your old tariff, new charges for your new tariff).
Here's a link that may help to explain the changes for you - www.o2.co.uk/afteryourupgrade