Do you know what exam board you do?
I do history A Level (year 12), and if you're on the same board as us (AQA I think), it is a MASSIVE step up
For GCSE, if you did OCR or similar, the biggest question was about 9 marks or so at the end of the paper. Lots of little questions. Analysing a cartoon, etc etc.
For A Level, there's two questions per paper 25 marks each. A source analysis question (two written sources are given, and the question is something like "using your knowledge of the time period and the sources, which is more useful to a historian studying the US's involvement in Asia from 1949-1960) and then a choice of two questions that will be something like "to what extent did Henry VII achieve his objectives from 1485-1509"
The MASSIVE step up here, really is the essay writing skills. At GCSE, imo, you can learn a simple structure and with good contextual knowledge get full marks. However at A Level there is SO MUCH emphasis placed on analytical skills which really are difficult to get to grips with. But don't let this put you off it is still interesting and with essay practice you will get there in the end
However, an A at GCSE will not necessarily equate to an A at A-Level. It could, definitely, but don't just assume because you did it at GCSE you're sorted for A Level; as long as you do all the work, read around the subject etc etc, there is no reason you can't do just as well. (But don't let your GCSE grades make you complacent!)
Any questions just ask