If a unoconv or ghostscript bug have prevented a document from being converted successfully and you upgrade unoconv or ghostscript to resolve that, there is currently not an option to reconvert the file. To solve for this currently, the student would have to change their document in some way and resubmit it, which seems too complicated of an ask and places the burden on the instructor to walk them through it.

A better option would be to offer, somewhere in the grading interface, a button that the instructor can use to attempt to reconvert the file.