New SIM Card

I am an existing ID SIM CARD user in my mobile phone but keep constantly using up the 500 MB Data allowance, resulting in having to buy add on data. It was suggested by a friend to buy a bigger capacity SIM CARD in the Black Friday deals being offered. I picked out the 4 GB data card with 1000 mins and unlimited texts. I did all the paperwork online and got a confirmation about collection from the nearest CPW store 2 days later on the 27 Nov. I took all the necessary paperwork as informed but was told that despite requesting WANTING TO KEEP the same number this was NOT POSSIBLE. This was because this was a NEW CONTRACT purchase and not an UPGRADE, which was the only way the DATA and the number could be seamlessly moved to the NEW CARD. The UPGRADE DATE on the existing ID card is 11 Dec, and as such was told by the adviser to return then and they would help out with doing the needful. According to the staff existing ID SIM card users cannot upgrade until the specified date. Can anybody please throw some light on this issue as I do not understand the logic of this??