It is a film from the early 2000’s about a teenage girl who goes to a mall and uses a machine to change her appearance (a bit like the game Sims), she then sees what she would look like as a boy, but then changes into him for real.

Answered question

are you sure it was a movie? was it in english? my first thought was “it’s a boy girl thing”, or even “hot chick”, but the mall simulation bit I am not seeing….
did she do a full body swap or just dress as?