The only thing I didn't like about the longer hair was that it came out of frickin' nowhere!
Otherwise, it was totally hot.
There was a reason for that.
Jennifer Lien had an allergic reaction to the glue which was used to fit the Ocampa ears on her head. So they came up with the idea of letting Kes have long hair, then she could hide the ears behind her hair and not have to wear the Ocampa ears.
In "Worst Case Scenario", Kes is short-haired again (or more correctly, the hologram of Kes has short hair) but if you look closely, she's not wearing the Ocampa ears.
It always strikes me as odd when actors have sudden allergic reactions to their make-up.
Yes, but that's obviously the fact here.
As for allergics, they can show up sudden and very unexpected.
And when someone is smeared in the face with the same goo for more than three years, there might be some unwanted and unexpected effects out of it. Obviously they didn't have the time (or didn't care) to find an alternative glue, instead they settled for the "long hair" solution.