The recommended position is the position at which the player's salary is the highest.
The fact that your player changed position from PG to SF means that his salary is the highest as SF.
The only way to control it is to train primary skills for PG and his recommended position will become PG again. But really, I don't follow the recommended position. They're just to give you an idea of where he could play, not where he must play.
You don't need to worry about it much.