post suddenly needing /tag/ (new permalink path)?

We’ve got 50 previous blog posts with the permalink

https://www.pstgroup.co.uk/sample-post/

Today we wrote a new blog post, #51, and despite being set to open access, it suddenly says people HAVE to login to view it.

Someone has just said this is because the URL is wrong and it should NOT be structured

https://www.pstgroup.co.uk/af5-march-2021/

as they’ve been for months but should be

https://www.pstgroup.co.uk/tag/af5-march-2021/

Changing the permalinks globally will KILL all my SEO that’s taken years to set up. Why has this /tag/ suddenly happened? Can anyone help me work out how to get around this?

Thanks

The page I need help with: [log in to see the link]