I'm a firm believer that you never demote someone. They lose morale in the short term and in the long term, they know they'll never be up for a promotion. It's better for all parties to fire someone and let him go elsewhere. If fired, Coach Gran can leave and pick a spot where he can learn from an offensive head coach or OC. He could also find an OC job on a lower non-power 5 or FCS level, work on his system, and re-prove that he's ready for another power 5 OC job.
This.
If CMS feels some sort of loyalty to Gran and is satisfied with the overall performance and direction of offense then promote him to AHC, give him complete autonomy to do what he wants with offensive staff, recruiting, game planning, and play calling.
If CMS is not happy with the offense nor the constant criticism and pressure, then make a damn change.
It's not fair to Gran nor the team to keep him on and bring another cook into the kitchen.