Prabal,
If invited to interview, you could provide the update at that time (that would be best). In theory you could also mail that information to the Admissions Office, but there is no guarantee that it will be received and processed in time before an interview decision is made in your case (if not already made).
More often than not, the qualities that led to a promotion should stand out in other application materials, as well, so ask yourself if the marginal benefit of providing such an update is worth it in your case.
-Victor
WG '11