It's important that your Change Notification forms make clear 1) whether or when changes are to be implemented, and 2) what the buyer's payment responsibilities are. I like the following model language for the two options that could apply:
☐ This form is a notification only. Supplier will neither initiate the [services / product] above nor invoice related amounts until a contract amendment is signed.
☐ This form authorizes Supplier to initiate the [services / product] above and invoice Customer [100% of CNF value] upon signature. Supplier will credit this payment against payments due under the eventual contract amendment.