Here are some of the key differences from upgrading via Amazon/Wirefly/LetsTalk/etc. as opposed to Verizon;
-resellers such as the above only have CURRENT carrier plans in their system, so if you are on any kind of grandfathered plan, you will lose it (IE, unlimited data). You essentially sign up for a "new" plan when you purchase through these places, even if you're simply picking the same plan that you already have.
-You enter into a second contract specifically with THAT reseller. Should you reduce or cancel your plan within 180 days of purchase, you owe that reseller an ETF. So, if you cancel your plan or switch carriers, you would own an ETF both to Verizon and Amazon. Again though, the merchant contract is only 180 days.