While it does clearly mention the following,
Please note that the Amazon Prime subscription/coupon codes can only be applied successfully by customers to an account which does not have a running subscription of Prime or Prime Shopping edition or any other such subscription. If you have an existing subscription, please apply the code to a new Prime account or wait for the current subscription to expire before applying the prime code. If you apply the code to an account which already has a running subscription, the coupon code will not work and cannot be reused.
Just wanted to check if someone has been able to extend their current membership by an year?