I don't have much experience of this personally, as I neither went through this process or did classics at A Level, but in general it's probably not a great idea. Assuming you did 4 subjects at AS, most unis only give you an offer based upon 3 A Level grades, meaning it's unneccesary to get up the grade for a fourth subject that you want to drop. I know a couple of my mates did this in Maths and Biology and they both didn't go up, whilst it took time away from the subjects that they were doing at A2. I got a D in Maths at AS, but didn't try and retake it and am now studying history at Southampton. However, at the end of the day do what you feel.