Change of plan

I need to change my plan to get more data, so I picked out from the SIM offers at the moment the offer for 4 Gig of data for the price of 3 Gig at £8 per month. All the paperwork was completed and confirmation obtained to collect the SIM card from the retail outlet 2 days later. At the CPW retail outlet I was informed that although I had indicated I wanted to keep my number I could not do it as this was classed as a new contract and not an upgrade, which is the only way to keep the existing number. My upgrade is due on the 11th Dec according to the existing ID plan. I do not want to miss the the special offer which according to the adviser could be the case, as it is part of the Black Friday offers. I have as such been advised to return on the 12 Dec to have this done. There was no indication of any such restriction when I ordered the SIM card. Has anybody got advice about how to deal with this problem?