DH and pitcher both bat

[JJ]

Here's one. OBR, using a DH. In the 7th inning the coach comes out and wants to bat the pitcher for the second baseman, and put any future pitcher in that spot in the batting order.
I know if the DH comes into the game on defense the pitcher bats in the spot of the defensive player who leaves the game, but this coach wanted to make the switch while they were on offense. Legal?