No. 1 domestic wireless carrier Verizon (NYSE:VZ) Wireless is tightening its device upgrade policy, making customers with two-year agreements wait a full 24 months before upgrade eligibility, up from the previous requirement of 20 months.

In an announcement today, the company said the move "aligns the upgrade date with the contract end date," which Verizon says is how long most customers wait.

Subscribers whose contracts expire in January 2014 will be the first affected by the change. Verizon reminded subscribers that they can purchase devices at unsubsidized retail prices at any time. Customers are still able to share upgrades with other subscribers on the same account, provided that both devices are in the same equipment category.

Verizon also said that credits earned under the older New Every Two program will no longer be available after April 15. The program ended in January 2011.