She's become an prominent member of Bajoran society, and she's had time to relax and enjoy life a bit more since her days living in caves. She's still tough and no-nonsense on the job, but she spends a bit more time on her appearance than before. It's a perfectly natural development. As to why the braids in particular, we can fanwank that it is a traditional Bajoran style, which would make sense for the character, though I don't recall any reason ever being given.
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