How to change carrier on iPad 2nd generation?
If you check the Apple Sim page for iPad here (Use Apple SIM with Wi-Fi + Cellular models of iPad), it says that you can use a nano-SIM card in the SIM tray to set up a cellular data plan with a different carrier.


  Japan: 
  
  
  AU, DoCoMo, Softbank (purchased in a carrier retail location): The    embedded Apple SIM and nano-SIM tray are locked to the carrier that
  sold you your iPad. Outside Japan, you can use the embedded Apple SIM 
  or a nano-SIM card in the SIM tray to set up a cellular data plan
  with a different carrier if you are outside Japan..
  KDDI (KDDI retail locations): The embedded Apple SIM is locked to    KDDI in Japan. The nano-SIM tray is locked to the carrier that sold
  you your iPad. Outside Japan, you can use the embedded Apple SIM or a 
  nano-SIM card in the SIM tray to set up a cellular data plan with a
  different carrier.