[johnnyg08]

I found an interesting piece in the 2009 MLBUM:

Crew Chiefs shall decide rotation assignments when a new umpire joins a crew. It is suggested that a substitute umpire who joins a crew from the Minor Leagues should start at 2nd or 3rd base. When a substitute umpire joins a crew from another Major League crew, consideration should be made of the substitute's last plate game. Minor League umpires should not start behind the plate when joining a crew from the Minor Leagues.