Hello,
You can upgrade on MY PHPDOCX page after login. On this page you can download the new package if you have an active LUS, or purchase the upgrade.
About your question, the same scripts of phpdocx 7 (and phpdocx 6, 5 and 4) will work perfectly. Since the release of phpdocx 4 some years ago, each new version of phpdocx only adds new features but the API doesn't change; you just need to change .inc to .php extensions when you include phpdocx (phpdocx 8 use PHP extensions instead of INC used by previous versions).
On https://www.phpdocx.com/news/post/phpdocx-v8-release-notes/214 you can read about the features added in phpdocx 8.
Regards.