For what it's worth: my wife and I both pre-ordered 6 Plus phones on Apple.com for our AT&T accounts, using AT&T Next ( as opposed to buying outright or upgrading w/ 2 yr contract ). We received the phones on launch day. My wife decided last night the Plus was too big and wants a 6. Like you, the closest Apple store is over 2 hours away.
At lunch today I went into my local AT&T store and they took care of it. They didn't have any stock, but essentially placed an order for a 6 and will call when it arrives - roughly 2-3 weeks. My wife will continue using the Plus until the 6 comes in and then I will take it to the store for exchange. I was in and out in 15 minutes.