You cannot simply add a new event format (you mustn't have two format definitions for the same event) and, as pointed out already, changing the current format would break virtually ALL current scripts. To safe yourself a single if-condition?
That aside the event has the current format because nickchanges have no location/target - you don't change a nick "in a channel" or in some way related to a channel. It's like requesting "on *:quit:<target>:" only because a user may be in some channel while quitting.