Would a PHP code upgrade for the cPanel etc. ruin an all html site (text and images)?

The control panel code has the ability to write to the website's directory and subdirectories, so it's possible. It's highly unlikely that a tested update would do that on just one site--to the point where I'd suspect deliberate tampering rather than an undetected software error if such an event occurred.

If you're asking whether it's a good idea to back up your site before doing something, the answer to that question is always "yes."