As far as I know it wont do any harm if you have completed all teh Quest on this Act. From what I have read when you complete an Act and go to the next all from the previous act is over with and you start out all over nothing carries over. Not really sure though maybe someone else will have a better answer for you but I dont think it will matter. Best thing to do is save before doing it so if you have to you can go back before you do all the killin and go on from there.
Good Luck to you and Fun gamin
J. W.
