Content models are foundational. They’re literally the framework on which your domain of content is built. And since every content object is based on a type, this means that some types might have dozens, hundreds, or even thousands of content objects that have been created from them.
Each of these content objects “fleshes out” the base skeleton of information that the type represents. Layered on the type’s collection of attributes are the actual values, which comprise that unique object of content.
This means if you want to change a type after content has been created from it, this is a lot like changing the foundation of a house after we’ve built the walls and floors on top of it. If the walls of the foundation define the shape of the house, what do you do when that shape changes?