At that point, you have until the end of this year to upgrade to a new phone and activate it, using the number you registered with Best Buy. You must sign a two year pact with Verizon, Sprint or AT&T when you make the purchase of your new phone from Best Buy or Best Buy Mobile. And like we said above, the $50 gift card can be used to discount the price of your new phone.
So who is going to be upgrading their phone in 2014? You know who you are. In fact, there should be a strong crop of impressive next-gen models coming out sometime this year including the Samsung Galaxy S5, the Apple iPhone 6, the LG G Pro 2, the HTC One 2 and the Samsung Galaxy Note 4 to name just a few. Everyone could use a $50 discount, so if you know you're going to upgrade, and you know that you're going to keep your number, why not take advantage of this offer? After all, this is one of those rare times when opening a new line is not going to get you the lower price..
Best Buy will give you a $50 gift card if you register your current phone number with them, and then upgrade that line to a new phone sometime this year
source: BestBuy via AndroidCentral