A good example of how discipline can change a career is Eric Thame, article in today's WSJ.
Turns out when he went to Korea, he was swinging at 33% of pitches out of the K-zone, after a couple years of a steady diet of off speed pitches (shades of AAA ball), he learned discipline and dropped that to 17% of pitches out of the K-zone - and so far it's paid off in Milwaukee.
Altherr turned his career around two years by becoming more disciplined, then adjusted his swing this year to be quicker - the combination is doubly effective b/c a quicker swing allows the player to wait on the ball longer and recognize its flight path.
Not every hitter can do so, but Rupp has so far this season and it's starting to pay off.
It obviously takes time, it's a different mindset, and there's going to be an adjustment period.
But learning to lay off bad pitches, set up each AB to get one or two swings at a hittable pitch, should pay off for a lot of players over time.