My Xterra is only 1 week old but I know I will want to upgrade the tires at some point

. Someone was telling me that you can trade in your current tires, and the best time to do it is just after you get your new truck. Is there any truth to this (for one thing i can't figure out the trade-in deal if you buy online)?
I can't really afford to get tires now, but the time i would be ready to do this would be in about 6 months - probably the worst time since my tires will have a reasonable amount of wear but no where close enough to need replacement. So if i would be much better off to get tires now then I could do it...
Any advice?
Thanks,
Harbones