If I change the settings for major/minor/patch updates, will this affect visitors who already have seen my CMP?

Yes, for example, the admin goes to the Admin Interface, selects major update. The visitor goes to the website and gives their consent. Since it is configured to the show banner on each major update, the visitor will see it again if a major update happens. 

Later, the admin de-selects major update in their Admin Interface (i.e. the banner should no longer be displayed, even after a major update). The visitor would see this change after 30 minutes from when he visited the page. That means, 30 minutes after he visited the page, he will have the new information that he should not be shown the banner anymore.

Do you need further help?

How can I get help with technical questions?