According to the video, this new mode hasn't been hiding since day one and was only added as part of a recent software update to the attraction. It makes one wonder what else might have been added to the ride, or what might get added in the future. At this point, it appears that "Chewbacca Mode" has only been confirmed at Disneyland, though one assumes that if it works there, it will also work at Walt Disney World. Although, it's possible that this new software update hasn't been done there, or hasn't been done yet. It seems that some cast member assistance was what clued these people into the hidden mode.