Go to Goods Shipment window and add 1 unit of this product.
Change the definition of the product as Service.
Try to change the definition to Item and Stocked again. You can not do that.
Proposed Solution If there is a warehouse/sales order document related with a product it should not be possible to change the stocked flag.
Stocked flag in Product window will not be updatable in case product is present in any order, invoice or warehouse document.
The old messages have been reverted because it could be used by external modules, so it could be considered as a functional API Change.
Create a new message instead and improve it to be more descriptive.