In reply to LABCAT.

Yes, patterns in use will remain in post/page content after switching themes. However, they will not remain for selection. Patterns are merely a specific grouping of blocks that already exist on your site. Theme authors can arrange blocks into certain patterns and present them to the user as an option to quickly insert into the editor.

The one caveat with this is that themes can style these patterns of blocks a certain way. It's very possible they won't look the same when switching themes. However, your content will remain intact.