It’s possible to change my visa in Japan?

Hi everybody! I'm not an English native speaker, so sorry for my English. But I really need your help and advices! I'll go in Japan for 3 months with a tourist /visitor visa, but I will like to apply in a Japanese language school for the April term. I know it's possible to change the visa before the end of the first one, but my problem is if this first visa expire, should I go back in my country? No prolongation? Thank you for your reply!

I know it's possible to change the visa before the end of the first one, but my problem is if this first visa expire, should I go back in my country? No prolongation?

Do you mean to ask what should happen if your residency expires while your change is processing? Immigrations allows a 2 month window for processing, and if I recall correctly this applies to those changing from temporary visitor to some other residency type. You just need to get your change application in before you residency expires, and they will put a notification stamp into your passport.