In my opinion it’s not a big step up especially if you’ve done further maths at gcse (if not still no need to worry). The pure maths at AS is in my opinion around the same level as what you need to do at gcse to get top grades. The stats and mechanics was new to me but stats is fine once you’re familiar with how to do it on the calculator and most the mechanics equations are in the formula booklet and the rest of the mechanics builds on what you would have done in pure maths.