As WJ points out, simply picking based purely on "best role" will result in a mess of a tactic. The role needs to fit into what you are trying to do as a whole. If one or two players don't ideally fit the role, it should still work. If half of the squad doesn't fit, then what you are trying to do doesn't fit the squad and you may want to rethink things.
Going from CM-A to CM-S isn't a huge adjustment. Its very literally changing the focus of the role. A defensive focus will better suit a player with slightly better defensive attributes, while a more attacking one will suit the better attacking player. But unless they are wholly one-dimensional players, switching between Attack, Support, and Defense should be fine.
A more overt example.... Using a defensive 4-1-4-1 formation but having a player who is naturally a AML, whether winger or inside forward. Ideally, I want to use a WM role for the ML. That will maintain the shape and defensive solidity. Its not idea for this player. For the player, I would switch the ML to AML, but that sacrifices the defensive shape. That means the player will contribute less defensively. Even if he doesn't have great defensive attributes, I want him in place to help out. So I will play him "out of position" and out of his ideal role, as an MR. I might switch the role to winger at times. But whether he's playing as a WM or a W, I would rather having him doing what fits the tactic, even if he can't do it perfectly, than doing things that don't fit the whole intent.
