An SMS message has been sent out to customers informing them of the changes made by the company. And as written on the text blast, the changes will take place alongside the first successful Auto Refill payment after June 8, 2016. With this change, customers on the $30 GoPhone plan are entitled to get the $5 discount.
As announced by the company, no further action is required to be taken by customers.
Source: AT&T GoPhone