There is quite a big jump in my experience but don't worry too much. I was in a fast-track group that did GCSE Maths early and we all got an A* without really even trying but the minute A-Level Maths started, it became clear that we weren't all getting consistent top grades anymore, especially for the people who started to discover which subjects they wanted to specialise in and focused a bit more on them rather than Maths. Just stay committed to it and you'll be fine, practise is key! A-Level Maths is also far more interesting because you learn about ideas that you literally never knew existed and it can be pretty cool. You'll do great!