It certainly could have been a voluntary “demotion” and there is a reason to believe he hasn’t changed a huge amount. There is an issue that needs to be resolved though:
On the one hand, he still seems to have a streak of violence in him the way he slapped Spock (at Spock’s request) in A Private Little War, which would track. On the other, you would think he would have worked with Spock in the plan to get Pike to Talos IV.
If we are to believe the two M’Bengas are the same person, which we have no reason to disbelieve (we can excuse the TOS M’Benga’s lack of an accent), an explanation for why he wasn’t part of Spock’s plan would be an interesting thing to ponder.