In short, yes. For most subjects (not maths, however, I don't think - I don't know how that works) to get an A* at A-level, you need an A average overall for the AS and A2 combined (80%, i.e. 360/400 UMS or 480/600 UMS, depending on the subject) and you need an average of 90% across all your A2 modules only. What that means is that you can go from a rock-bottom C at AS (60%) and, if you get 100% in all A2 modules, you average 80% and, since you got more than 90% at A2, you get an A*.
In other words, how high or low your A was at AS has no bearing on your A* chances, so no need to resit. Hope that helps!