It appears that I saved my presentation in a read-only format and now I'm unable to "save as" to edit or use the theme for future presentations. Is there a way to convert this document "back" to a readable, editable file?

Have you tried right-clicking on the file, selecting "Properties" and changing it's attributes there?

I just tried that. What may have happened was that I marked it as "Final" presentation to make it a read-only, not understanding the would "lock up" the file.