I would think them holding a Smash tournament with Smash Switch so close probably means the mechanics between Switch and Wii U are near identical.
At least some form of character reveal or trailer will probably be shown now as well.
From the Nintendo E3 Website:
So they will be running the Switch version.Join the party as players from around the world battle in this invitational tournament for bragging rights, for fun, and for glory on Nintendo Switch!