Time is a key factor in rewriting a document or a project. There may not be enough time to rewrite the document, however, it is reasonable to just modify some details in the document to make the document more appealing. As a technical writer, you need to weigh the time you will spend writing a new document against the time you will spend modifying a prepared document. This will determine if rewriting the document is the better option. Rewriting document is not always compulsory, especially if the document is free of error and concise.
A document that is missing key details, full of errors, or lack clarity needs to be fixed, however, the writer may waste more time starting the document from scratch to editing the key details in the written document.