Aww,Patchy gone back to her white outfit. The black one is so cool, why did she change!
Mostly because like a normal person, one doesn't typically ALWAYS wear the same thing everyday, even taking into consideration how fandom sometimes depicts Patchouli as a shut-in pajama-wearing NEET. This Patchouli is a more sociable and better adapted person on the whole, and doesn't always wear the same thing as time passes. This is a time-skip montage, after all...