Which exam board? If you're doing AQA then it's likely you've already studied half of the content for GCSE (I had already done Rivers and Population) but there's just a little more extra detail and it's conceptually more challenging, but only slightly. I didn't have any more case studies to learn for AS than I did at GCSE, but I think we just got an unnecessary amount at GCSE. The style of questioning is more demanding and you have to know what you're talking about, but I think your B demonstrates you've already gained a good level of understanding. If you're willing to work hard to expand it, go for it.