On the Posts menu for a single page site, there is a ‘convert to blog’ link, which immediately converts to a blog. This is nice to have accessible – but it does not seem reversible if one accidentally clicks. I ended up manually re-making a non-blog site and changing the DNS over to that to deprecate the now-blog old site.
Would suggest
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a confirmation prompt before moving a non-blog to a blog. especially if (2) is not possible/
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a downgrade to non-blog (preserving theme, css, uploads and pages, but removing posts; prompt to download backup like deleting a blog)