Yes the jump is rather big. It's not necessarily difficult, it's just a lot more time consuming. I mean, most A Level subjects require you to be proactive and doing out of class research alongside homework. At GCSE, it was mostly have a lesson, forget it, then come revision time, memorise it again. With A Levels however you're expected to know the facts and understand them.
But schools tend to make the process slow and steady. The first two weeks they might stretch you with loads of homework, but that's normally to get rid of time wasters, and soon after it gets a lot more manageable. If you expect a jump, you probably won't notice it. If you didn't expect a jump, you'll definitely notice it.
I know languages (especially languages!!!) and essay subjects tend to have a huge jump compared to sciences, since you automatically have to writing AS standard essays from day one.