I actually don't know about that. I've just seen her in pics, so I didn't really get a close look.
Anyway, maybe Sesame Workshop (Children's Television Workshop, at the time) had the same idea Bill Watterson had when he created "Calvin and Hobbes." The first strip showed Calvin catching Hobbes in a tiger trap because he wanted to establish how those two got together. Later on, he felt it was unnecessary so later strips seem to indicate that it never happened. I guess it was the same way with Sesame Street. Oscar became famous for his green coloring, so in later eps there was no mention of Oscar being orange. That's probably why it was never explained how Gordon lost his afro, or how Big Bird's appearance and voice changed or anything like that. Kids who watch the show today wouldn't have been aware of these changes, so they feel it's not worth mentioning. Though it would be nice for us Muppet Central fans if every once in a while they referred back to the history of the street or how characters looked when they were 'younger' or stuff like that.
By the way, does anyone know of the dialog between Flip Wilson and Oscar or whoever he was talking to on the show that explained his color change? Again, I think I missed it when I saw it on You Tube. Just my curious mind acting up. Lol.