I think it's a double 'no' here!
Firstly, pay-as-you-go phones are normally 'locked' to a particular network. (Pay-as-you-go phones are usually cheap for this very reason. i.e. the phone company subsidise the purchase price so that they can get you committed to using their services).
Secondly, you seem to be assuming that there's more information on the SIM card than there really is. Yes, it will contain your phone number and phonebook entries but that's about all.
Think about it: If you want to check up on a friends number you just press a few keys on your phone and up comes the information without the need to make an actual call. On the other hand, if you want to check your credit balance, you type in a phone number (or call it up from memory) and press the 'Dial' button. Your phone then dials O2's computer and their computer returns the information to your phone.
In short, your phone book entries are on your SIM card but your credit balance is on O2's computer. So, even if you could get the SIM card to work, you wouldn't be transferring the balance when you transferred the SIM card.
Sorry!
Chris