I don't do A level French but many of my friends did, and I've heard it's basically a different subject compared to GCSE. Even the similar bits are much harder.
In terms of the other three subjects, I do them currently and I didn't find them much of a step up. AS Maths shouldn't be a problem to anyone who got an A* at GCSE (A2 is where the big step up is), and Biology is basically the same but different/more advanced topics.
Chemistry was a bit of a strange one - initially I was getting Cs, Ds etc and even in my AS mock I got a C. However, during the final weeks it all seemed to click together and doing past papers helped incredibly - ended up with 96% avg and now doing A2.
Top tip - do past papers as much as you can and learn markschemes. Make sure you leave lots for closer to the exam, but if you know what they're looking for you can boost your grades fairly easily. Other than that, just make sure you understand EVERYTHING about a topic before you move on - if this requires going to help desks then do it!