Jason Baker kommentar är korrekt.
Cuarón also made the characters feel a bit more contemporary via their wardrobe. In the previous films, the students had worn school uniforms and robes the majority of the time; in Azkaban, there was a shift. The young characters were now seen in normal, everyday clothes as a further reflection of their personalities and maturation. Too, they could individualize their uniforms a bit
Harry Potter: From Page to Screen: The Complete Filmmaking Journey by Bob McCabe - Page 104 - Prisoner of Azkaban analysis