This is a common scenario for technical writers: there is existing content, let's say for V1 of a product, and the same content needs to be provided for V2. Do you limit your work to making changes to the existing content for technical accuracy with V2? Or do you start with a blank slate and design a new doc, generously copying content from the old where applicable?
Here are some of the issues I consider when making that call. The first two are fairly cut-and-dried, but after that it's more subjective.
Even when (1) the product isn't changing that much, and (2) you have plenty of time, and (3) there's really no feedback or data, and (4) you have definite ideas for improvement -- you still want to consider whether reworking this doc (rather than just updating) is the most important thing you could be doing with the time you have.