Shouldn't be a problem at all.
1. Remove the seat.
2. Remove the staples from the '03 cover you are going to use.
3. Next you will need to insert a long blade screwdriver into the elastic straps so you can pull the small wire out of the elastic straps, and not have them snap back into the seat.
Repeat 2 - 3 on the 2004 seat to hold the straps, and reinstall the wire.
then center it over the foam, restaple from the center to the outside. 1/4" or 6mm staples I think??? (same size you took out)