virgin sim query - car phone warehouse

[cliffpage]

my wife and daughter went to carphone warehouse today and bought virgin nokia 5530 for £129.95.
they thought the deal was 129.95 if you bought £10 airtime at same time (ie 139.95 total). they were charged 129.95,
the man took the old virgin sim from my daughter's old phone and put it in the new phone, but said do not lose the new sim card in the box as you will need that when your old sim stops working in the next few hours or next few days.
So at present the new phone has the old sim card in. the credit shows as £11, although prior to visiting shop the sim had £1 on it. So it seems we got £10 credit but did not pay for it.

The receipt mentions Virgin Prepay Upg Topup £0.00, and Virgin Prepay Upgrade NOK5530BLACK £129.95.

This is not a monthly contract nor was the old phone which was bought elsewhere as pay as you go years ago.

What I don't get is the bit about the old sim will stop working in next few hours / days, and we will then need to put the new sim in.

Why can't we just carry on with the old virgin sim card so that we keep old phone number.

My wife and daughter are a bit clueless on mobiles and sims (I don't claim to know much but I would have challenged him a bit more on what was going on if I had been there)

Can anyone explain?