How I can upgrade to "protect plus plan" without having to entirely discard my already paid £24.99?

Hi. I am using your ring.com basic plan service in the UK at this moment.After subscribing to your basic service by paying 24.99 GBP per year since 5/27/2020, today I just realized that this plan does not include the automatic alert system I experienced during my trial with your "protect plus plan".However, when I tried to upgrade my service from the "basic plan" I am in now, I was not able to pay only the difference but had to face the full 80 GBP per year amount seen on my PC screen.I haven't paid this yet as I did not want to pay additional 80 GBP although I already paid 24.99 GBP for my basic service.May I know how I can upgrade to "protect plus plan" without having to entirely discard my already paid 24.99 GBP for my basic plan?I would appreciate if this matter can be resolved with urgency please.Thanks.