At this time Royal Caribbean has no plans to replace the Monarch in Los Angeles. If you are looking for a quick three or four night cruise to Baja California after October 2008, your only choice is Carnival.
It will be interesting to see what the future brings. While Royal Caribbean is building three more ships (Independence of the Seas will start taking passengers in 2008 and the two Genesis ships are scheduled to start sailing in 2009 and 2010) Royal Caribbean has also announced that the entire Sovereign class (Sovereign, Majesty and Monarch) will soon leave the fleet (I think over the next two or three years). The Empress is older than the Monarch or the Majesty, so its days at RC are likely limited. That just leaves the six Vision class ships to do the short cruises. Will one of the Vision class ships come to Los Angeles to do the short three and four night Baja California cruises? Time will tell. However, I would not count on it happening before 2010.