"...but now she gets up early." It sounds very strange to reduce the second phrase.

1) I used to play the piano, but now I play nerves. A) I'm not sure what you're trying to refer to by saying, "but now I play nerves." Is nerves an instrument? If you were to say that to me I'd be very confused because when I think of the word "nerves" I think of someone's mental state. 2) Kate used to get up late, but now early. A) It would sound better if you were to say, Kate used to get up late, but now "she" gets up early. Also, you could imply that Kate used to get up early by saying, "Kate get's up early now" or "Now Kate gets up early." ~ Keep in mind I'm not a grammar professional or anything, but as a native English speaker this is how I would say these sentences.

I used to play the piano, but now I play nerves. Kate used to get up late, but now SHE GETS UP early.