@Taafe unfortunately the only way this can done is over the phone. There are extended waiting times currently, but you will likely get through eventually.
The other way around it would be to upgrade the accounts online to either a new device or sim only. This will take the full device plan payment online and change you to a new tariff. Moving to sim only is almost always cheaper than remaining on a refresh airtime tariff, but if you wish to remain on your current tariff you will need to contact customer services to clear the device plan balance.
Hope this helps.
I work for O2 but my comments and opinions posted here are solely my own and do not reflect those of Telefonica.
