Do you want the Anime reason, or the corporate reason?
In the Anime, he upgraded/changed his wardrobe because he was basically trying to 'start over', and it was meant to illustrate that he's going at it with a fresh slate (or a new identity if you go by the Comatose Ash Theory).
As far as the corporate reason (aka the real reason they probably did it), somewhere along the line, the writers probably wanted to reinvent Ash. The wardrobe change is presumably meant to symbolize this 'new Ash' as opposed to the old one (from Kanto and Jhoto). Coincidentally, he also got discreetly taller at about this time.