I'm having a similar problem my top up anniversary was yesterday so I presumed I could change my numbers after my £5 was deducted for the next month, however I have visited my 02 numbers yesterday and today and it would appear that I still can't change them. Any ideas?
You can change Your O2 Numbers once every 30 days. You can do this by visiting o2.co.uk/youro2numbers, by texting SETUP to 21300, or by calling 2425 free from your O2 mobile (see point 8 for details).
If you don’t select your full entitlement of ten Your O2 Numbers, you will still be charged the full rate, but you can add to the empty fields by visiting o2.co.uk/youro2numbers or by calling 2425 free at any time.