In Vimwiki, a header level can be decreased, by pressing
Is there a native Vimwiki command to remove a level from all headers within a visual selection?
Keep in mind that Vimwiki supports a number of formats (e.g.,
=== Header level 3 ===,
### Header level 3). A search/replace, or a custom command, or a plugin might be a solution. But I am curious if Vimwiki already supports this out-of-the-box?