Demo for this is out now on the Japanese eShop.
I played a couple of minutes of it and I'm just glad to report that the puzzling action is still a butter-smooth 60fps on the lower screen. I was worried the addition of 3D RPG battle visuals up top would cause issues.
I am a bit disappointed though that 1) cutscenes don't appear to be realtime, rather rendered using the in-game graphics away from 3DS, and 2) the game doesn't support 3D at all. Sonic Team's Puyo games all made incredible use of 3D, particularly Puyo Puyo Tetris.
I wonder if this will be on the Switch as well?