Apprently, you can even install the electroluminescent gauges after the fact ... see
http://forum.mazda6tech.com/viewtopic.php?t=119
And you can obvisouly easily (although at great expense) swap out the front and rear bumpers/fascia. However, getting the fog light from the light panel into the non-Sport-Package 6 is probably more complicated, or at least no one's bothered doing it yet. What else ... yeah, the wheels and tires, in 2003 only the SP got the 17". In 2004, of course, that's a Premium package addon, unrelated to Sport Package, and all V6 get it anyway as well. Lots of people are always selling theirs on ebay or mazda6club's For Sale sections.