It was the older look that made her my favourite Gym Leader. She actually looked like she'd throw you from heaven on high just for cocking an eyebrow at her.
Now she just looks like she'd ask for your number.
For the whole clothing change, I've always attributed it to her time in the anime. Because at first, she was a psychotic bitch, and her outfit showed that. I like to think that her Haunter giving her a better personality caused the clothing change.
Natsume: "I-it's my first time, so I don't know if I can do it properly... Is that okay with you?"