I don't see any change in the possibility of Carl replacing Bo based on staying at YSU or going to BGSU. If he improves BGSU (and he will, good grief look how bad they were) it will only make a return MORE likely.
Regarding BGSU being a lateral move, 6 letters covers that FBS vs FCS. The BGSU non-conference schedule for 2018 includes Oregon Pac12, Maryland BIG10 and Georgia Tech ACC. Ours includes traditional football powers Valpo and Butler. I know we are all homers, and big "MVFC is best" etc. etc. but we are 1AA not 1A and we are clearly a level below. I am 100% opposed to trying to make the commitment to move up as entirely too costly, but we are not "top level" football either. Consistently coaching against Big10 ACC Pac12 and of course the MAC is definitely a step up for a coach. (In addition, BGSU has Illinois, Notre Dame, and Ohio State scheduled the next couple years)