My son already has a mobile phone with ID which is now up for upgrade.

He wants to keep his old phone and number, which I guess is paid off now?

How do we keep the old number and phone but switch to a sim only deal?

We are currently getting 10Gb for just over £20 month, if I were to start an new contract with sim only I would only be paying £6 per month for that same.

I cant find where in the account section I can change to a cheaper sim only deal now its out of contract