It is possible that there will be an option to toggle between old and new graphics akin to StarCraft: Remastered (where pressing “F5” switched between the original and remastered graphics at will). This may not be possible, however, due to the complications of toggling between a sprite and 3D-based engine. However, it is intended that players will be able to switch between the different graphical settings, even if it’s not instant.
With that in mind as well as the fact that there’s cross-play and slaps you with a Warcraft 3 CD key. I’m going to make an absolutely absurd impossibly off the wall guess that both versions of the campaigns will be available with the new art.