Some can be modified, others not. There are limited choices. When you get into the "More Commands" part, you can select one of your commands and if the Modify button is active, you can choose a different icon.

See also my reply to your parallel post.

To really make good flexible changes, you may need to modify the Ribbon using XML (not through the Word interface, no flexibility there). Customize the Ribbon (It doesn't take rocket science)