[enonod]

I discovered that I had mis-tagged books as B and decided that they should be A.B. A did not yet exist. I selected some books and edited the metadata adding A. in front of all the Bs. It worked brilliantly but was laborious.
I was unable to find a way to do this under Bulk changes without deleting and recreating Tags, so I used single mode.

I also tried amending a book with A.B and then tried to drag B in the Tag Browser to sit under A but that is not possible.

I looked for a way to amend the Tags alone (Tag Editor) without using the Books hoping that by changing the Tags hierarchy, the books would follow automatically. No.

I am hoping that somebody can show me that such a thing does exist, and I missed it, ready for next time.