Here is something you can try.
Firstly you will need a new and unregistered 02 free PayNgo sim card.
1. Call 5450 from your existing no. & handset
2. Press option 1 and they will give you an authorisation no. & let you know how much you'll be due as credit to your EXISTING no.
3. Put your new sim card into your phone & dial the same no. using
this 'new' sim and select option 2.
4.They will then ask for auth code (given on the original call), your EXISTING phone no.,your name, your postcode & house number.
5. They then confirm the amount to be credited in 14 days & advise that the NEW SIM CARD no. will be disconnected straight away.