I mean someone could get literally full marks at AS and still end up failing the A-Level - it doesn't matter how you've done in the past, but what you do now to improve. Of course, if you work in exactly the same way you did before to achieve the B, then you are unlikely to get an A*, but if you change something, whether that be how you revise, or how much you revise, or re-teaching yourself previous topics you didn't understand, etc, then getting an A* is definitely possible.