To update product categories for a product, I recommend you to export the list of product categories first to create their xml id.

Then export all the products for which you want to do a mass update with the columns that you need to update (import-compatible export). When you export products, they will get an xml id too.

In the csv file, fill the new product category with the corresponding xml id. Keep the product xml IDs in your import file, so they will not be created again, but only updated.

For other fields such as gross weight, you can just enter the values. No need for xml IDs in that case.