I have done that before. It worked but its easier to start from scratch and just d/l cars seperate and put them in the correct location.
All you have to do is change the names of the files and put them in the correct 2003 order. And tune using Madman's excellent Tweaker to make sure the helmets are in the correct position.
Its easy as hell but hard to explain.
The order I used is the following :
2003 team ----------> Replaces 2001 team
Ferrari ----------------> Ferrari
Williams --------------> Mclaren
Mclaren ---------------> Williams
Renault ---------------> Benetton
Jordan ----------------> Jordan
BAR --------------------> Sauber
Sauber ---------------> BAR
Jaguar ----------------> arrows
Minardi ----------------> Jaguar
Toyota ----------------> Minardi
Team 11 -----------------> Prost
And the helmets are numbered so you just place the correct helmet on the correct car number.
hope this helps.