While Jesse McCartney has confirmed voice acting for a localised version of KINGDOM HEARTS HD 1.5 ReMIX, and Alyson Stoner hints at returning as Xion, forum member Taochan emailed voice actors Vincent Corazza and Richard Epcar asking if they would be reprising their roles as Zexion and Ansem Seeker of Darkness respectively. Unfortunately, neither could confirm their involvement with KINGDOM HEARTS HD 1.5 ReMIX.
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While they did not confirm their involvement in the game, this does not completely remove the possiblity of them returning to voice their characters. It could mean that they don't want to break their NDA(Non-Disclosure Agreement), that they have been contacted by Square Enix but haven't started voice acting work for the character yet, that they haven't be contacted by Square Enix yet, or their character doesn't have any spoken roles in the game.
KINGDOM HEARTS HD 1.5 ReMIX releases March 14th 2013 in Japan. There is no confirmed date for other regions.