I'm one of the guys who didn't need to change my nut when I went to a high C on my Curbow. I didn't even have to tweak the truss rod. Granted, it's made of Rockwood, and has two graphite bars, and two truss rods, so I guess you could say it was stable.
I currently have a set of TI Jazz flats strung E-C, and it sounds absolutely amazing.
When I took it to Curbow to have the electronics overhauled about six months ago, I asked him about changing out the nut. He said if it was playing okay, and intonating okay, then it didn't need a new nut. So, I would try it with the current nut, and if it works, leave it alone. But, if you are getting nuttiness, or buzzing, swap the nut out.