Warcraft III: Reforged "will be compatible with the SD version"
Art lead Brian Sousa clarified this in our interview.
Warcraft III: Reforged was revealed last Friday at the opening ceremony at BlizzCon, giving fans a chance to see the classic RTS with improved graphics and some slight tweaks, but art lead Brian Sousa said in our interview with us that the good news also extends to SD players, who can play with those in Reforged.
"The HD version will be compatible with the SD version, so any changes that we are making for Reforged will be reflected in the SD version, so if you have a version that's remastered, Reforged, and you're playing with a friend that didn't buy it yet, you can still play, and all of the mods and everything will continue to work. We're not changing the base game at all, we're basically building a rendering engine on top of it, so should be exactly the same."