When the Lumia 928 was first released, there was a spec sheet online at Verizon that mentioned that Lumia 928 phones will have the capability to connect to XLTE when the service goes live via firmware update.
What is the time frame of this update being pushed out/released???
Verizon has to wait for Nokia/Microsoft to do this. Once Verizon gets the software release from N/M they can test it and push it out to 928 owners. Nokia has been pretty good with firmware updates in the past, excluding that ugly period during the N95's heyday where North American customers got ignored. Now, however, with Microsoft running the show, 928 owners are at Redmond's mercy. Verizon can to nothing but wait.
To expand on what 47seijar said, most likely not till Windows Phone 8.1/Cyan is released to the devices over the next few months.