I don't believe he pulls a Simmons, but I could see him sulking and taking it personally as a demotion rather than a better use of talents. I could just as easily see him being talked into it and owning the position too.
As it stands now, the coach was overruled by the FO so it's a moot point.
I don't think highly of JBB. I keep hearing about how players love him, but all I see is terrible coaching. I'd expect if they loved him, they'd buy into his gameplan. But until game 1 of this year, I've seen little to suggest that they have.