New sim Old phone

[Kate131]

I've spent years on Virgin Mobile and it was easy to swop numbers/sim.  Give notice on rolling monthly sim, purchase new sim.  Swop sim and away you go.

I'm out of contract for my current 1st 02 phone and I thought I'd buy the current offered £10 a month sim, give notice on the refresh rolling contract sim, swop sim and there I am, old phone, new number.

I can see no way of giving notice online (please don't answer with some convoluted guide link that doesn't work) so I telephone to ask.

It appears 02 don't really want me to do this because there doesn't seem a yes you can do this?  

Why can't I buy a cheaper monthly cost sim and use it in my current 02 phone and cancel the current sim?

I just don't understand what is complicated about this?  I have no attachment to my current number.  If Virgin have never had a problem with doing this, why would 02?

[MI5 · Accepted solution]

@Kate131 

You just need to upgrade to a sim only tariff in your MyO2. 
Hope this was easy for you and not too convoluted?