to stay for his 5th year and decline his options to go into NFL draft or transfer to another school for his 5th year. Isn't that standard operating procedure?

Also consider that Lea was promoted to DC about a week before the deadline to apply for the draft. So that would logically give rise to a 2nd conversation about Tranquill's role while he still had time to decide (if Drue felt it was important). Despite Kelly being an a-hole, I think the idea that he pulled some kind of Machiavellian bait-and-switch is kind of unlikely under the circumstances.

The most logical conclusion, in my amateur opinion, is that Drue probably doesn't believe that a move to LB hurts his NFL chances.