Can I exchange an iPhone 4 with an iPhone 5 without getting a new SIM?
According to this article, the problem isn't just the size.


  It turns out, as well as a smaller size, the Nano SIM is reportedly
  15% thinner than the Micro SIM cards. So even if users cut the Micro
  SIM to Nano SIm size with a knife or nano SIM cutter tool, the Sim
  card would still be too thick to put in the iPhone 5.