Thanks Chris, Ben (& DanteC for asking the exact question I had, that didn't seem to have an answer in Help)
I just switched from PAYG to 3GB, Unlimited mins, keep number, PAYG balance transferred for £6 p.mo. (12 mo. contract. Black Friday deal, normally £10 p mo. ?) by calling the number Chris gave.
No new SIM was necessary.
I can't understand why this transaction wasn't possible from my account page.
👍📱