There are three selectable types of product – single version, multiple version and options product. These are explained further in @4.1.4. Product types.

When you select the product type, some of the dropdown menus immediately below will be enabled or preset to certain values and disabled. This is because certain selections of these only apply to certain product types.
If you are editing an existing product, and find that you are not permitted to change the product type, it is most likely because the product has associated data which is incompatible with the other product types. For example, if you have a multiple-version product, it cannot be changed to a single version or options products without first deleting the extraneous versions.