You can re-index a document revision that is not the latest revision. For example, if a document has three revisions with the third being the most recent, you can re-index the first revision.
The revision process is the same as Re-Indexing Documents, only you would access documents from the Document Results Search List using the Revisions/Rendition option from the right-click menu. When the Revisions/Renditions dialog box is displayed, select the oldest revision and re-index as needed. All options are available during a regular re-indexing, including the ability to change the file type.