Product Model/Family

Hi

I've assigned a Product Model to an Equipment record without an Inventory Item and would like to change the Product Model however it won't let me. I've also Created an equipment record with an Inventory Item with an Item Master Product Model category that pulls through to the equipment, which i then try and change on both the Equipment and Item master record wanting it to update the Equipment record and it won't save the change. Is this standard functionality, is it a bug or is there another way of updating the product model or product family on an Equipment