With my 6S order, I was told that I'd have to mail back my 6 once I received my 6S. When I got my 6S, there was no return envelope/box/pigeon included. I was then told that I'd have to go to the store. When I went to the store, they told me I had to ship it back but were "generous" enough to give me a prepaid shipping label.
It's always rocky with preorders, returning phones, upgrades, order updates, shipping updates, etc. I guess one day we'll all get used to the madness and these threads won't go for thousands of pages. Just look at last year's pre-order thread. LOL