Actually, I think this would be fairly trivial, as MSDN has its own installer on the VS2003 DVD, and all the MSDN related files are in their own subfolder -- VS setup just launches MSDN setup and exits, MSDN installation is not part of VS installation.

That said, you should be able to just delete all the old contents of the MSDN subfolder and put in the contents of MSDN April 2004.

Of course, editing the DVD ISO is the pain...

Regards