TL;DR - Just return your 6 at BB. Then go to apple store and see if they can process a return for your 6 Plus and repurchase that same device with a contract. Make sure you check that your eligibility has been reset. Or else it may give you headache.
Also why not just buy it as device only? I mean the phone will be unlocked out of the box and you can use on different carrier if you later plan on traveling.
I believe you are going to get a lot of different answers. Because each store does it differently. When I went to AT&T store and I talked to the manager about few days before launch day as I was having issues trying to place an online order for the 6 Plus as the apple.com couldn't connect with my server.
Anyways I asked the manager about this similar situation and she told me that if you buy a phone full price at the AT&T store or even adding a line just to get the phone and then canceling the line afterward, and having to add that phone back into your current line, they can't do it because once you return the phone, the system marks that phone as a return device and then marks it as unsaleable. So they can't resale that device. And have to take that back.
Now I'm not sure if this is true because it was really busy at the time I was there and I did not want to waste anymore of my time, so I just left it at that. But of course this was at the AT&T store at my location. IT may be different for every store.
I'm not sure how apple works on this, but you might have to go and experience it for yourself and see. I've heard some say that it is doable at the apple store but YMMV.
