I did AQA Biology for GCSE and I've just completed WJEC AS Biology with an A. I had an A at GCSE but even with a B it shows that you're very capable of the content so it just means you need to put a lot of hard work in because the workload is so much more. I know loads of teachers etc will say it's a big jump from GCSE to AS/A2 and it really is, especially for Biology because you need to memorise so much work. It's worth looking through the specs for the topics you'll be covering to see if you'll enjoy it. I personally really enjoyed them. If you think you'll enjoy it and are willing to put a lot of work into it, from the start then you will get a good grade. (I thought I'd drop a grade at A level too but once you understand the amount of work you need to put in, it'll work out fine!).