Honestly, I don't really know. You have to either talk to Sony or Verizon to find out whether or not that phone is compatible with the Verizon network. Verizon doesn't use GSM, like most companies (Cingular, etc.). Verizon uses CDMA.
The GSM networks uses a SIM card, which allows you to switch hpones whenever. Read more about it here:
From what I've read about CDMA, it is possible to switch serivce to the new phone without going to the store, but the new phone has to support CDMA.