I just topped up £10 then changed tarif from the £5 one to the £10 big data one thinking it wouldn't switch me unless I had £10 in my balance and I've lost £5 credit when it switched me over, I don't want to have to top up another £10!
As soon as you change tariff it bills you so when you topped up £10 it will have taken the charge instantly.
If you want to check this with Customer Services then you will need to contact them on 4445
Current Phone: Sony XPeria Pro-I and Motorola Razr 5G I also have a Planet Computers Gemini
I work in IT as an Enterprise Messaging and Collaboration Tools Administrator (Office 365, Slack and much much more) and manage hardware both Mobile and Desktop / laptop, my desk is a bit like PC World
-- We are all customers and dont have access to any o2 accounts