You can switch phones through vzw.com. You click the device you currently have, and within that page there is an option to switch it. You type in the IMEI of the new phone and off you go. You could also call the 800 number and switch it that way. Switching the SIM card for phones on Verizon won't work, you'll need to call them or use their website.
MAKE SURE your new device is a CDMA Verizon branded device. An AT&T S2 will NOT work on Verizon no matter what you do.