While what you say gives hope, it seemed pretty clear that this was an intentional and overt change. To quote from: Midnight Alpha Test Development Notes - #7 by Linxy :
"Fully collecting a class set’s appearances now unlocks all lower difficulty variants. For example, collecting all Mythic appearances will automatically unlock the Heroic, Normal, and Raid Finder variants while unlocking Heroic would only unlock Normal and Raid Finder, etc. Eligible sets will be marked as such in the set collection UI. This change is retroactive to all legacy content. Items will no longer change their appearance when upgraded.
Developers’ notes: While we feel the Catalyst has been largely successful in expanding access to powerful seasonal set bonuses, it has also generated friction in its dual use as the primary source of class set appearances for many players. This friction has been further exacerbated by loot roll restrictions in situations such as participating in LFR raiding. In conjunction with this change, we want to better preserve the value and identity of item art from difficult content."