Hello,
A license can be upgraded only to the latest release. You can upgrade a license on MY PHPDOCX page after login. All packages include all files, so you just need to extract the ZIP file to use the library.
About upgrading the package, you can point from one version to other changing only the path to phpdocx, the API method names don't change (albeit a lot of new methods and new options have been added since the release of phpdocx 7). Please note that phpdocx 8 changed INC extensions to PHP extensions (https://www.phpdocx.com/news/post/phpdocx-v8-release-notes/214), so you need to include CreateDocx.php (not CreateDocx.inc). On https://www.phpdocx.com/news/ you can read all new features added in each new version of
Regards.