Read forums to see many exchanged 6 Plus for 6. I don't think it was possible for anyone to exchange 6 for 6 Plus (unless they're on Sprint http://www.seaturtle.org/idevice/iphone6_grid.shtml).
I'm pretty sure you can. If purchased at Apple store you have 14 days to return for full refund with no restocking fees. Then you turn around and purchase the 6 Plus, if they have stock of course. Or you could buy online or anywhere else.
Of course that's IF you want the 6 Plus. I have one. At first it was way huge. Got a case on it and much better. Not slippery anymore and I can use it one handed for most things. Fits in my pocket and is sooooo nice to have that screen. Absolutely love the Plus now and will be keeping it. Takes a bit to get used to but so does the 6.