Odds Prewett actually starts? Seems low. Wouldn't Newlan go over him?
Snyder will only play them on special teams. Shelly will start and play every snap unless he gets injured. 
Odds?
Prewett over Newlan if you're looking at the rotations thus far, but this probably bumps Newlan from 3rd string to second string.
Shelly will start and play the majority of snaps unless some circumstance happens where Prewett jumps in there and just kills it like Green did against Auburn.
I'm not sure I'm following your logic here...
Assuming the depth chart WAS:
Green
Shelly
Prewett
Newlan
Removing Green moves Newlan from 4 to 3.
Barnett - Prewett or Newlan
Shelly - Prewett or Newlan
We handle safeties interchangeably, right?
So, before, it would have been:
Starter (Green, Barnett)
Shelly
Prewett
Newlan
If we were to do a wholesale change, it would have been, before, something like:
Barnett/Prewett/Newlan
Green/Shelly/Whowever
So, fine, he's gone from 5th to 4th. But 1-2 is first string, 3-4 is second string, 5-6 is third string.