Here’s what Blizzard mentioned about the patch in a press release:
Celebrate 20 years of Diablo by traveling back into the depths of the Cathedral where it all began in this upcoming Diablo III: Reaper of Souls content update. Get ready to explore a monster-filled dungeon 16 levels deep, and come face to face with the four main bosses from the original game—all with retro touches like special graphics filters and eight-direction character movement.
The patch will be available for free to both the console and PC owners of Diablo III: Reaper of Souls. It will go live on public test next week.