Strictly speaking the line rental saver is not the line rental contract. It's a method of buying 12 months line rental credit up front. Think of them like vouchers with one being used each month. They are not for a set amount so the price change doesn't matter. You still have a normal line rental contract in place with a minimum term but are covering it each month with these vouchers.
BT are notifying you of the line rental price change as it will affect you when your current LRS comes to an end. You will then have to buy another LRS at an increased rate or pay standard line rental at an increased rate.