If you are capable of getting A/A* in the subject at GCSE, then you are capable of getting an A/A* at A-level. However this doesn't mean you will, just that you can. If you don't work hard then you could very easily get a U, as a level is far more reliant on consistent hard work.
I know people who get all A*s at GCSE and flopped at A Level... Equally, I know people who got Bs/As and got AAB/AAA. It's about putting the work in and not under-estimating the content. It's a step up and as long as you keep up with the work and ensure you revise you should be fine.