It's not buffed. What Konami release is Earth Heroes (both Dian and Solid Soldier are Earth). Mask Change will NEVER be limited, period. But for Anki, they may consider hitting it if the new Earth Heroes fail to compete with Dark Heroes in the meta
You don't need Clayman. And really, unless you're directly building around Masked HERO Dian, I wouldn't even use any of them aside from Solid Soldier. Clayman's worse than Woodsman and Wildheart anyway, so you shouldn't use it even if you needed another.