I had originally intended to just do a picture of a Original Changeling and a Reformed Changling side-by-side in Generation 1 art style, but as I was working on it I began to imagine a G1 mythos for Changelings.  
Seeing that Changelings were not reformed until G4, Season 6 episode 26 then I certainly did not want to place reformed Changelings in G1. So if Changelings had appeared in G1 then they would have certainly been unreformed.  
I imagined slightly different mechanics for G1 Changelings. Most noticeable are the red eyes and the fact that they come in 3 different types; Earth Pony, Unicorn and Pegasus rather than all Changelings sharing traits of all 3 types of Pony.  
The reason for the red eyes rather than blue is because, evil. In the 80’s evil creatures almost always had red eyes. Blue would make them seem potentially friendly by 80’s standards.  
The reason for 3 types of Changeling is also because of taking old-school writing into account. I imagine that if Changelings were in G1 then they would not be born Changelings. Rather Changelings would be created when a Pony was drained of all their love and essence, leaving them hallow, insectoid creatures craving to feed on others. Instead of being reformable the condition could be quickly reversed by either The Rainbow of Light, or by The Flutter Ponies.