I tried activating Mask Change when I was dueling and I couldn't. I think you guys have to update the card text where I can activate Mask Change when my monster is face-down. Also, I couldn't activate it during my opponent's turn. I have the mirror 3 YGOPro which was given by GruntLord.
Mask Change targets one "HERO" monster. A face-down monster is not a "HERO" monster, so you won't be able to target it. The "(its original Attribute, if face-down)." part of the text only applies if Book of Moon or something is chained to your Mask Change and flips the target down.