Hello Eric,
The short answer to your question: procurement-related changes should go to the Control Procurement process.
To help you understand, it depends on the nature of the change. For example, if you are managing a house construction project and you have contracted out the gardening work: If the change is related to the house construction (ex. color of the bedroom), then the approved change will go to Direct and Manage Work. Alternatively, if the change falls in the gardening work (ex. the contract), then it is a procurement-related change and goes to the Control Procurement.
Hope this helps,
Joe