From your Inventory page, select 'Update Existing Products' from the 'Bulk Manage' drop-down menu. Select 'No, I need to download a list of my products'. Select 'Manufactured' and 'Purchasable' from the 'Product Attributes' section. Build and download the Product sheet. Set the 'Manufactured' column to 'True' and the 'Purchasable' column to 'False' for each product that you would like to convert. Click here for more information.