Description of problem: A topic name is defined in the content spec and in the topic itself: If you want to rename a topic, you need to change it in three places: topic's content specs, the properties tab of the topic, and the title in the topic content. Considering that right now there is not a way to figure out which content specs are using the topic, this can become quite a challenge. I believe it should be possible to change the name of a topic in one place and have the change distributed wherever necessary in background.
One thing to note is you should never rename the Fixed URL Property (hence the name "Fixed"). We've left it editable as a courtesy, in the case that you really want it changed, but in reality it should never be changed once a book has been published. As for the content specifications, currently that would need this RFE first: https://bugzilla.redhat.com/show_bug.cgi?id=831910
As long as the topic's XML is valid, the title element in the section is always set to the title property. There is no need to update the title element; if it is missing it will be added, and if it is present it will be overwritten. You can also use permissive processing with the Content Spec Processor with the -p or --permissive option. This will remove the requirement for an exact title match in the content specs. See https://docspace.corp.redhat.com/docs/DOC-91733 for details.
Just to clear up the above comment - title element is the <title> xml element in the <section>, and title property is the property set on the topic.