Ha. I doubt it. I mean really, people who ordered the 6+ have no idea what they're getting themselves into. It takes time. When I got the Note, I told myself I would give it 13 so that I could return it on the 14th. During the first week I was convinced I would return it, but by day 14th that wasn't the case.
Well I don't plan on returning my 6+. Hell, if the 6+ hadn't been announced, I would've surely went back to Android and gotten a Note 4. Between my job and especially college and all the media that I watch on my device...bigger screen phones just work for me. I've never owned a Note before (or a phone that's in the 5 inches range) but I think I'll do alright with the 6+ That display is gonna look awesome. As is the battery life and the extra camera features.
And yeah I could see some people returning the 6 and getting a 6+. Just like you had S5 owners who returned it for the Note and Note owners who ended up returning their devices for the S5. I've seen iPhone customers who've returned their device for an S5 b/c they wanted a bigger display. So 6 owners returning it for a 6+ b/c they want the bigger screen, better camera, and battery life is definitely possible.