Early entry for student visa

Hi,

I am moving to Japan for exchange semester from a visa free country (can visit japan for 90 days as a tourist without a visa). I will be getting a student visa for my stay starting 1. february. However I’ve been planning to do a few weeks of travelling in Japan before my exchange starts. Is it possible to change my visa from “visa free tourist -visa” to student visa inside the country, or do I have to fly to Korea for a few days before my exchange studies and student visa start 1. february.

Thanks for help and sorry if this has been posted here earlier since I’m new here.