Coupon for active members


I am wondering if it is possible to offer a coupon code for active members. I have a member that is on our second tier pricing, but I want to offer them a discount to match the pricing of the first level.

Is it possible to create a coupon and apply it to their account that is already active. They signed up on 8/4/16, so the account is active.

If that won't work, is there another way to change the subscription amount?