While Shuichi Ikeda and Banjo Ginga will return to play as Char and Gihren respectively, Mayumi Tanaka (LUFFY of One Piece) and Megumi Han (Gon of HunterXHunter) play the young versions of Char Aznable and Sayla Mass respectively.
Seems Sunrise is making a shout-out to One Piece. Luffy and Shanks as the FREAKING Red Comet. Instant-awesomesauce!!
That said, the movie will be out in 13 theaters across Japan on 2/28 for 2 weeks. On that day as well, watchers who happen to be in those theaters to watch the OVA can preorder the Blu-Ray for this movie.
Let me reiterate: LUFFY AND SHANKS IN GUNDAM, TOGETHER!!