Jschutz, I may have missed something in your post or sig, I don't think I did, but 12k is a low down slap in the face in these parts for a trade in on a '03. If that's what the dealer is willing to give, and I've got my info right, sell that thing private party. Heck, I'LL buy it for 12k! Furthermore, you should get at least 10-12 grand private party with 400k on the clock, and IMO that's a ridiculously low estimate.
I'm a firm believer in "if it ain't broke, don't fix it". If my vet or farrier showed up in a old beater I might make fun of him but I can guarantee no one touches my livestock but the guys that I know and have a working relationship with, I don't care what they're drivin'. Trust in your clientèle, if you've got skills, they'll stand by you. Then when '09 rolls around your current truck will be paid for, if it isn't already, and the kinks will be worked out of the 6. 7's. Keep the old one for reserve or sell it then.