Not saying this is true... Not saying all parties would be OK with this... Just saying...
I read somewhere that Grier can opt to sit out all of 2016 and keep his 3 remaining years of eligibility. (EDIT: 3 years to play 2 seasons according to source).
So hypothetically if Treon were to take over and keep the job, he'd be the starter for the rest of 2015, all of 2016, and all of 2017.
Then in 2018 Grier would be a redshirt Junior.
Again, grain of salt...
EDIT: Found the source:
https://www.sbnation.com/college-foot...rs-quarterback