When I got mine I picked out the bike, the seat, and the tires and they put it all together for me.
We're allowed to bring our bikes in for a tune-up before the 30 day mark...I'm kinda thinkin' that doesn't include anything more than a quick look-over, but if we came in during the day, they might be willing to do a tire swap for a small fee. Only catch is that *that* shop carries a limited range of tires...may not have what I want, and I dunno what they'd say if I brought in some I'd bought elsewhere.