Nintendo has reconfirmed Mario's new voice actor will be revealed in the credits of Super Mario Bros. Wonder and there won't be any special announcements happening ahead of this release.
Speaking to IGN at Nintendo Live 2023, Nintendo of America's president Doug Bowser reiterated how there wouldn't be any name reveal until the game launched and also mentioned how there would be no formal announcement prior to this. Here's exactly what he had to say:
"we'll let that play out and that'll be within the credits, people will learn who the new person is at that point in time, but we don't plan to make any announcement in advance of that"
A previous statement from Nintendo asked fans to "please wait for the game to be released" to find out who the new voice actors are for Mario and Luigi.
Doug Bowser also mentioned how Charles was apparently "very excited" about his new role and how Nintendo looked forward to having him "continue to represent" the Mushroom Kingdom as Mario moved forward.
Unfortunately, Martinet doesn't seem to completely understand his new ambassador role - recently noting at a convention how he's not exactly sure what it will involve just yet and claiming he'll be finding out alongside fans.