To get upgraded to LTE, your Motorola XOOM must be returned to the factory
Other information in the leaked material shows that shipping and handling will be entirely free and the upgrade will be available for a 6 month period from the time that LTE becomes available on the tablet. Verizon reps are being told to sell the 4G experience for the Motorola XOOM by telling customers that 4G will increase the speed of the device by 10 times while songs will be downloaded in seconds and movies in minutes.
Verizon is telling customers to head to www.verizonwireless.com/Xoom4GLTEUpgrade and enter their email address so that they can be notified when the 4G upgrade will be available for their tablet.
source: Verizon via Droid-Life