Most stuff in hex-editing isn't that simple, I am afraid, there could be multiple copies of the same stuff, redundant stuff which are useless, and the most likely reason is that you haven't updated the relevant data in the .dbi files to correspond with that. The .exe file is very difficult to edit, since the file is so big, and picking single lines to edit might just turn up useless. Moreover, in hex editing, (especially large complicated important files....e.g. .exe), there could be a checksum value hidden somewhere, and if that is the case, that would be nearly impossible to find.
So, all you must do is edit, try, test, edit, try, test and try to understand.
The best way to learn is from the .dbi files first, IMO, because that is where all roster data comes from. I haven't found an actual use of the team data inside the fifa2003.exe, since it's different from what it was in fifa2002.exe.
Keep trying.
. I editted some things, but I didn't saw them in the game!