Same holds true here. Easier to get into a P5 conference when you are already in a P5 conference.
BYU needs to do whatever it can to get it. Even if that means taking less for 8 years (hopefully not the case but can't rule it out).
If and when the Big 12 implodes, BYU is already in the club. As long as it has performed reasonably well (and assuming no more than the usual fighting over the affiliation with the religion) BYU will land in a P5.
Just get invited to the first party...worry about the after party later.