Currently Stock Items and Non-Stock Items are stored in the same database table by maintained in two separate screens and changing an item from a stocking item to a non-stocking item does not appear to be easy. A user should be able to change the ItemType from the UI in order to change an item from a stock item to a non-stock item and vice versa.
If it is a non-stock item moving to a stock item, then validate the required info is set.
If a stock item is changing to a non-stock item, it should only be allowed if no inventory is on hand and all relevant documents have been released and the item is not referenced on an open sales order or purchase order.
In other systems, the stock and non-stock is simply a checkbox on the item master allowing one to toggle between the two (assuming no stock on hand and nothing on sales order, purchase order, etc.)
I know that new items could be created in some cases instead, but when that is done, BOMs and Product Configurations, etc. all need to be updated which is a somewhat painful process.