Unfortunately it isn't so easy to swap between PAYG and Pay monthly or Pay monthly to PAYG.
You basically have to transfer your phone number off the TPO network (ie to O2 or Vodafone) and then back again.
You'll need a new SIM card:
Another network SIM card: Purchase and activate another network SIM (ie pick up one from the supermarket)
Once you have your SIM card - if you'd like to keep your phone number, you need to transfer your current phone number out to the other network and back in again.
You'll need to port out and port in your number:
1) Contact us to let us know you'd like your PAC code (when you use your PAC code your account will automatically be closed/cancelled - you're charged per calendar month, if you don't think you'll get this done by the end of this payment run you may wish to wait till next one so you make the most of your spend)
2) Call up "new network" and give them your TPO PAC code - typically this can be done the following working day.
3) Once ported over, ask your "new network" for your PAC code
4) Activate your new TPO SIM card
5) Contact us and give us your new TPO number and the PAC code from your "new network" and we'll arrange your number to be transferred back in
Sorry - it is a bit of a palaver, there unfortunately isn't an easy way around it at the moment.