I thought that the friend code was randomly generated and based on what ds it was in. however, when I accessed wifi on my ds lite with the mkds cartridge that I accessed wifi with on my ds phat, the fc stayed the same, even though it asked me if I wanted to access wifi cause it would erase previous settings.
since the friend code is not even created until wifi is accessed, if a save is uploaded before the person used wifi, it should not do anything different at all.