G4 obviously has no connection to G1 outside of them reinventing characters to be used in the series. The Smooze was a giant green snot blob in G4, how is that connected to the Smooze from G1?
So in my extremely humble opinion, the "evolved" Changelings could be G4's version of the Flutter Ponies. Either that, or Rarity gaining those butterfly wings is how you get turned into a Flutter Pony.