[mgjordan]

If you have a turned left hand grip, you have to hold off the rolling of the forarm on the downswing to keep the clubface open enough to hit accurate shots. If you move your grip into a vertical position, you have to teach yourself how to roll the club on the downswing. The "no roll" with a turned grip is a compensation that is required to keep the face square. Changing your grip requires you to change your clubface control.

Also, most players who have a turned grip tend to bend their left wrist during the backswing. If you adopt a vertical grip, you may need to work on keeping the left wrist flat during the backswing.