If that specific expansion happened, why would we expect to have to move just Auburn to the East? Why not move Auburn and Alabama east and Vandy west to join OK and A&M? Here's how the divisions would look:
East: Alabama, Auburn, Florida, Georgia, Kentucky, South Carolina, and Tennessee
West: Arkansas, LSU, Mississippi State, Oklahoma, Ole Miss, Texas A&M, and Vanderbilt
That would protect the following rivalries- Iron Bowl, Auburn-Georgia, Florida-Georgia, Florida-Tennessee, Tennessee-Alabama, LSU-Arkansas, Egg Bowl, and Tennessee-Kentucky. I might be missing some, but that protects most of the big games I can think of. It would also add natural and pre-existing rivalries with Arkansas-A&M and A&M-Oklahoma. LSU and A&M could become a good rivalry based on proximity. LSU-Oklahoma would become the potential division deciding game every year. I could see Vanderbilt-Ole Miss becoming a big rivalry just to see who won't finish dead last. It might also strengthen the West in basketball by sending Vandy West. If you had six division games plus three out-of-division games, that still leaves room to for a standing out-of-division opponent and three out-of-conference games.
I'm no conference commissioner, but that alignment makes sense to me.