What is the risk of changing my user account's "full name" mac os x? Why the warning of "might damage this account"?
There is no danger in changing the user's full name in this dialog.  Some of the settings on this pane could mess things up. For example:


It's possible to change your shell to something that doesn't exist
Changing your user id can mean that you can't access your files, since they are owned by the old id
Changing the short username might affect all sorts of weird things, since many apps may depend on your username staying the same.