Before we begin, let me reiterate that NOTHING IS OFFICAL. So there is no need to panic yet. But the possibility of this game happening in 2017 does have some question marks.
Vanquish the Foe wrote an article in 2015 about the contract details of the Mississippi State BYU series. In it there is language that would protect Mississippi State if the SEC were to go to nine conference games OR protect BYU if they were to join a Power 5 conference like the Big 12.
BYU notified the Big 12 that they are intrested in joining the Big 12 in all sports or as a football only school. If BYU were to join the Big 12 in 2017, then this game along with other games that they scheduled for 2017 can be cancelled. And with BYU already playing LSU in 2017 in Houston, this game is more likely to be dropped.
The Big 12 currently plays nine conference games and has not released the 2017 conference football schedule. So adding four members by their first year of their conference championship game maybe ideal. The Big 12 plays a 9 game conference schedule and BYU already has rivals Utah and Utah State scheduled for 2017. The Hawaii game could still be kept as a thirteenth opponent.
Are the Cougars likely to join in 2017? Probaly not because they already have a full 2017 schedule. But it certainly is possible to get out of those games, and if the Big 12 really wants their teams by 2017, then BYU doesnt have much of a choice. However it is more likely that BYU joins the Big 12 in 2018 or 2019 when their schedule is not quite as full.
In my opinion, this is a legitimate concern that the Mississippi State Athletic Department needs to be aware of just in case the Big 12 announces BYU as a member in the next month or two. However it could also be an easy fix as Wisconsin is scheduled to travel to BYU in 2017. If BYU opts out of the Wisconsin contract along with Mississippi State, then Wisconsin and Mississippi State can schedule each other and fulfill both of their conference's Power 5 scheduling requirement. Since Wisconsin was orginally planning on traveling to BYU, and MSU was originally planning on hosting BYU, then Wisconsin traveling to Starkville only makes sense. But a neutral site game would not be the end of the world either. Wisconsin's only open dates in this scenario are September the 16th and the 23rd. Both of which are open for MSU as of now pending the 2017 SEC Football schedule release.