It is a custom for F1 teams to start a season with a redeveloped car of the previous season. Ferrari was the first to introduce this concept and last season McLaren perfected into a art form (because their new car wasn't ready).
According to the Italian magazine "Quotidiano Nazionale" Luca di Montezemolo (Ferrari CEO) is demanding that Scuderia
Ferrari starts the 2004 season with a brand new car. "It's pointless to redevelope the F2003-GA, the car has acomplished it's mission". The new regulations which will be enforced next season require a radicaly different car , to redevelope the F2003-GA would be too time consuming and too costly. Developing a new car seems the logical option.
If they do start the new season with a new car then McLaren will have a headstart because they had the entire 2003 season to test their new car (The MP4/19).
No word from Williams BMW yet as to what they are going todo but if things continue like they are now then McLaren Mercedes will start the 2004 season with a overwelming headstart.