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Does this plugin convert all words in the title?

No. There is a user-configurable list of stop words, pre-populated with a host of common words, which can be used to fine-tune the resulting tags.

Will this plugin overwrite my existing tags?

No. The plugin checks for the existence of tags, and if there are none, writes them based on the title.

When are the tags added? At publish or save?

Tags are added to the post when it is saved, whether as a draft, update or by publishing, but not when the post is autosaved.

Is there a way to reset the list of ignored words back to the original?

Yes. Simply delete the current list of stop words, and Title to Tags will replace the list with its default stop words collection (stopwords.txt)

Will this plugin put tags on my old posts?

Maybe. It will not handle the process automatically, but if you open any untagged post and save it, Titles to Tags will work.

Does this plugin work with versions less than 3.1?

The most recent release of this plugin was designed to handle 3.0 and above. It has not been tested below 3.0, however, there is a branched version in SVN that will work with versions greater than 2.7: http://plugins.svn.wordpress.org/title-to-tags/branches/2.1/trunk/