BYU supposedly has language in all of their game contracts allowing them to cancel if they join a P5 conference, but I wouldn't be surprised if there was a notice requirement, likely at least 12 months, for them to exercise that provision. Given that BYU would have to either move or cancel at least eight games for the 2017 season, and given how far in advance OOC schedules are usually set, I would be very surprised if BYU could start in the Big 12 in 2017 if they weren't notified until October.