I mean this in the sense of try new pitches or make mechanical changes.
I think that pitchers and coaches are always making adjustments. I think coaches prefer to tackle smaller adjustments during the season and larger adjustments in the pre- or post-season.
A lot of the in-season adjustments are made to set the pitcher up to make some bigger changes down the road. A couple of small tweaks that put the pitcher on the right path will make those later adjustments not seem as severe.
Of course, there are plenty of high school coaches that don’t have a clue about how to make pitchers better.