You have to keep it activated for at least 60 seconds
(well, that is probably how long it will take to go to the activation screen on your vzw.com account and input the new meid
There is no need to keep it there. I regularly upgrade one of my lines but for another, so I activate, then immediately change that line back to the original, then activate the new phone on the other line. I've even had them do it in the store once as I was picking it up (however, they took an hour! Mostly as they are slow as ..... I shoulda ordered online!!!)