Earlier a substitute fielder wasn't allowed to keep. But from the last rule changes (including no DRS reset after 80 overs, DRS in T20 etc), this was also implemented. Virat utilised full advantage of this and bring better keeper, DK.
@Gobinaath bro, A substitue to a fielder is allowed but I'm not sure whether a substitue is allowed for a wicket keeper.
Mean while it seems like a great come back by India. Actually it seems very refreshing to see our Indian bowling attack coming back strongly after a big partnership. May be this is a new culture which is developing.