PITCHER MOVING TO A DEFENSIVE POSITION

Rules 5.11, 5.10(b):

If a game pitcher is switched from the mound to a defensive position, this move shall terminate the designated hitter’s role for the remainder of the game. The pitcher just removed from the mound may bat in the designated hitter’s spot in the batting order; or, if more than one defensive change is made, the pitcher may bat in place of any one of the substituted players (manager shall designate the place in the batting order to the umpire).
EXAMPLE: Home Club relief pitcher Jones pitched the top of the ninth inning. For the top of the tenth inning, Jones moved to first base and relief pitcher Smith entered the game as the pitcher. Jones was placed in the eighth spot in the order where the first baseman Carter was hitting and Smith was slotted into the third spot for the designated hitter Anderson.