Yes, the DA could re-instate the charges. Having charges dropped is not a determination or adjudication of them; therefore, the ban against being tried twice on the same charges is not brought into play. It's unlikely the DA would do this--dropping the charges probably represents the DA's honest evaluation of the strength or worth of the case--but the DA would still have the opportunity to to bring charges. This puts a premium on the alleged assailant keeping his or her nose clean and NOT doing anything which would prompt the authorities to revisit the issue and reinstate charges, possibly with additional charges, for a later assault or infraction, added as well.