What exam board are you on? I can only speak for OCR because that's what I did for GCSE & A level.
I found AS chemistry very much like the C7 GCSE module, just in a lot more detail and you actually have to understand it more. Yes, it is a jump from GCSEs but then all subjects are.
My advice is make sure you understand things as you are taught them, I know some people who didn't and then struggled when it came to the papers. If you know it well enough to explain to others then you're fine. Also do all the past papers you can get your hands on because at least 50% of the paper is fairly standard & predictable.
I took Maths, Physics, Chemistry & English Lit for my ASes and Chemistry was probably the easiest one of all (though I am very much into science) to be honest.
Good luck!