There have been rumors about whether or not if Martinet would continue being the voice of Mario after people noticed some of Mario's lines in Super Mario Bros. Wonder sounded a bit different from what people are familiar with, and that Wario in the new Warioware also sounds different from what little voice clips we've heard. Martinet is up there in age (he's almost 70) and when you compare his Mario voice now to how it sounded back in the 90s or 00s you can tell aging is beginning to affect the voice, and unfortunately death is an inevitability so even if Martinet kept voicing Mario for more years to come he'd inevitably have to be replaced. Given Martinet is still going to be working with Nintendo representing the Mario IP though I would assume he's probably going to train an apprentice, or likely already has, to take over the role (I highly doubt Nintendo would just have Chris Pratt take over as Mario in everything, he'll probably just stay "Movie Mario" while the games either have one new person or multiple new people take over as Mario, Luigi, Wario, and Waluigi).
As of making this OP Nintendo says they'll be releasing a video later featuring Martinet and Mario creator Shigeru Miyamoto addressing the change as well.