Upgrading plan from Basic

I purchased a Ring Doorbell 2 and the free trial plan ran out, so I purchased a Basic plan and all went well. I just installed 2 x Floodlight Cams which are still running on the free trial. I want to upgrade to the Protect Plus plan, before the free ones run out. I went to the website and there was a button to click to Upgrade Plan, but when I went to pay for it, they wanted the full price for a new Protect Plus plan, so I aborted the transaction. How do I go about upgrading and not having to pay the full price, only the upgrade price? I'm in the UK. If I have to call Customer Service for this, what is the best number to use? I can call the USA (for free) if necessary.