Thanks, Deborah, but that wasn't quite the question. However, I did figure it out. Yay, Sky, indeed!
I didn't want to move stuff down a level, but split the contents into two levels. So instead of Gen 5:5 in one level, I wanted Gen in level 1 and 5:5 in level 2. Translations are set up so that Gen becomes {001}Genesis, with the numbers hidden to force the sorting.
1. I Saved As with a little different name, just in case things went south, and set the index to have 2 levels of subs. I'm still using Old Testament and New Testament as mains.
2. I grouped on the first book, Gen. Then I used F12 to demote all of Sub1 to Sub2. Sub1 was then empty.
3. I used Find and Replace in Sub2 only to find Gen and replace it with nothing.
4. I selected the column of Sub1, typed Gen in the first record, and hit Ctrl + Alt + Enter to propagate the change to all records.
Voila! I will have to repeat with each book, but it's a lot easier than placing the cursor in each record and demoting the chapter and verse.
And now back to the hard grind of the subject index 😭