I took AS geography with no knowledge of GCSE - I got full marks in one exam and an A in the other, it'll be fine!
Even if you had done GCSE (some were doing it simultaneously because they were accelerated students in year 11), you wouldn't have had any advantage on our board - you just had to learn all of the case studies anyway, and the physical stuff bore no resemblance to GCSE.
This might be different, for instance, if your school is doing AQA Rivers and the Cold Environments or Coasts options, since these are covered at GCSE - some students might have a leg up on river profiles and whatnot. But I have a revision guide which covers all of these topics and it doesn't look that hard to learn. I actually did learn the rivers, cold environments and food supply issues topics from the book before I knew which school I was going to do geography at, and they were fascinating! But no prior knowledge was needed, and it's not that hard, just lots to memorise.