Z:gnu-www-ja-gpl-faq--8fda28-=3Ca href=3D"=23Consider"=3EConsider t/en

Consider this situation: 1) X releases V1 of a project under the GPL. 2) Y contributes to the development of V2 with changes and new code based on V1. 3) X wants to convert V2 to a non-GPL license. Does X need Y's permission?