Tariff change or contract cancellation

Hello, I currently have a contract with O2, it ends in April and I am about to cancel the renewal.

I would stay with O2, but only if I can change the tariff. I have the app, and when I look at swapping tariffs, it doesn’t show me the tariff I want. But when I search separately, I can see the tariff and choose it. If I chose it, does it just add it to my existing tariff, which means, I would have to pay 2 tariffs (old and new) or replaces it completely, so, I only pay the new one? What I currently have is the: O2 Blue Basic and the O2 Blue All-in XL. I don’t want that anymore, I want to get the Samsung Galaxy S10 with O2 Free M Boost with 20 GB a month. How do I do the swap? I basically want a new phone + 20 GB data per month.