Name change and flight tickets

This summer im going to Japan (i live in the Netherlands). The tickets were booked a while ago, but within 1-3 months from now i will (hopefully/probobly) change my name and sex on my official documents (im transgender).

Does anyone know if i can just call or e-mail to let them change my name on my ticket or do i need to have an extra document to show i changed my name and sex? I have no idea what the rules/policy for this is.