Verizon recently changed their upgrade eligibility. Instead of being eligible to upgrade every 20-months on a 24-month contract, you are now required to wait the full 24-months.
"In alignment with the terms of the contract, customers on a two-year agreement will be eligible for an upgrade at 24 months vs. today's early upgrade eligibility at 20 months. This change aligns the upgrade date with the contract end date and is consistent with how the majority of customers purchase new phones today. The first customers impacted by this change are customers whose contracts expire in January 2014. As always, customers may purchase a new phone at the full retail price at any time."
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news.verizonwireless.com ··· ent.htmlIf you purchase a phone at full retail pricing, you can upgrade at any time during your contract and the plan / contract end date will not change (as you are not signing a new contract or changing existing terms on the contract).
It is my understanding that you can use the new "Device Payment Plan" to purchase the device at full retail without changing the contract terms.