I have a SharePoint site that is configured with FBA and AD authentication. Currently, the Forms Site is extended by the AD site. Due to various factors, I need to reverse this, so the forms based site is an extension of the AD site.
How would I go about doing this? The side is full of content, so there is no margin for error. Will un-extending the web application cause any data loss?
Firstly backup your system. (and be sure you can restore it.)
Secondly, unextending your web application should have no effect on the content, as the extension is just an interface through to the content.
Thirdly, I'm not quite sure what you mean: "reverse this, so the forms based site is an extension of the AD site." Are their two sites? where you want the forms site to become a subsite of the AD site? or are they two site collections or web applications?
You may want to look at site migration tools, which should be able to do this relatively seamlessly.