Her K2 was added the day after the first episode aired, which would've given them more than enough time to actually get her K2 uniform in when the episode in question came.
I think they gave her the proper equipment but kept her original uniform was because her K2 uniform was based on a repaint of the ship done just prior to the Solomon Islands campaign, and the anime only covered up to Midway, so her having her original uniform would be correct.
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