Players never, ever, ever, ever stop tinkering with their swing or pitching mechanics. It is a constant process. When you hear a team/player talking about off-season mechanical changes, that is a major overhaul (think Jason Heyward) not tinkering. JBJ's swing when he's going good is fluid. His problem is both mental (trying to uppercut everything) leading to mechanical (going from a slight uppercut to a loopy swing with a bad bat path). It's not a problem he can simply work on in the offseason since it's an in-season problem. The only one who can avoid those bad habits is himself, and he has to work on maintaining his adjustments during the season.