Do you need to use the Inspector to change element types dynamically? The following demo allows users to drag elements from a stencil palette to the JointJS paper, and then dynamically change the element type via the Inspector.
Select any element on the paper, and then use the dropdown menu in the Inspector to choose the element type.
The source code of this demo is available as part of the JointJS+ commercial license. Don't have a license yet? Start a trial and use the source code of this and many other demos for free, with no obligations, for 30 days.