When adding a circle I can see the 'Add circle' panel below the tool bar, but once I do something else it disappears. How do I get this back? How can I change the number vertices on an existing circle for instance? It appears that you have to do this immediately after adding the primitive, otherwise you're scuppered?
Creation is special. Some options you are see are valid at creation time only. That is standard Blender behavior. The options panel will disappear as you move on to the next mode or task. If you created a with sphere 32 rings and then in [edit mode] removed many faces ... it is difficult to understand what it would mean to change the rings to 7.
It is difficult to get a creation panel behavior that is acceptable to a large consensus after some object has been potentially modified.