There have been lots of transformations over the years, but the oldest one that really sticks out to me as "transformation" driven is one most of you probably won't recognize. There was a short lived Comic Strip cartoon Tiger Sharks in the US. It's way too "poof" by standards now, but I think it may be the catalyst point for me.
https://en.wikipedia.org/wiki/TigerSharks
https://youtu.be/7vKtlvPINSo
There are others that had single event transformations, such as the Elisa in Gargoyles a few years later that shifted it from interest to fascination but this was the first one that really clicked. (I discount the more tame transformations of He-man and She-ra, not to mention the transformers / gobots.)