Reshape is a Blender add-on that lets you modify the attributes of geometric shapes like cylinders and spheres after they have already been created. You can adjust parameters such as segments and rings right where you are, without starting over. If you set the wrong number of segments on a cylinder or didn't get the ring count on a sphere quite right, you can change them directly.
The add-on also includes a backup option that saves your original mesh state, so you can revert to the initial configuration whenever you need. Version 1.3.4 added an option to choose where the add-on is displayed, between the Edit and Tools panels.
One point to keep in mind: Reshape does not technically modify the existing mesh. It copies the information from the original mesh to create a new one, so attributes applied to vertices or edges are not inherited. The new mesh is generated from the original's local axis information, which means it may not function correctly for shapes that are not aligned with the local axis.