Peggy has evolved quite a bit since the beginning of the series, having gone from a rather mousy secretary to a single, fun-seeking career woman enjoying city life. She undoubtedly earns more as a copywriter than she did as Don's secretary, so she can afford somewhat nicer clothes, and I'm sure she's gotten a few style tips from Joan along the way. Here's a promo photo for the upcoming season from the official show website:
This is certainly a more sophisticated look for Peggy, as is this one:
Both of these are great. They remind me of photos of my family that I've seen in old albums (Mom, do you remember anything like this?), and also of pictures of 1960s performers like Diana Ross and the Supremes.