I could be wrong but I think because of the computer 03 maps won't work on 04 and vice-versa. At least thats what I remember the guy form Dyno-Jet telling me, then again I don't have a great memory either.
03' maps will work on 04's and vice versa. I have had a 04' PC and a 03' PC on my 04' and run 03' maps. It's all the same. The factory changes between 03' and 04' are gone once you take off the stock can and get a PC.