My local costco says they will let me keep my current unlimited data plan by transfering my upgrade eligibilty(on UDP line) to my second line(2gb tiered plan). Then, they will switch the phones. But a local bestbuy says i can transfer my upgrade on UDP line to another line(no UDP, 2gb data plan), but i would be losing the unlimited data. Costco seems to know what they are doing, but what is the difference here? Activation methods seem same.