AX 2012 - Project ID in a purchase order

According to the “New, Changed, and Deprecated Features for Microsoft Dynamics AX 2012” article, under the What’s new in Project Accounting section, it says that:

You can assign a project ID to any sales order, purchase order, service agreement, service order, or service subscription. Previously, these types of documents could be associated with a project ID only if they were created from within Project management and accounting.

Does anyone know anything about this? I haven’t been able to do this. I created a new project, and then a sales order inside that project. I then ran master planning for the item in my sales order, and planned purchase and production orders were generated. I firmed the planned purchase orders, but then when I open the purchase orders, I cannot assign the project ID. The Project ID field is uneditable. The article said ANY purchase order. So far this doesn’t seem any different from 2009…

Any thoughts or suggestions on the matter?