Thank you: that works! In a way. What happens is that the list itself is updated (when I look at it in the gallery, it shows it starting with e.g. 4), but the heading numbers in the document don't change (they stay at 1).

The gallery preview of the "Numbered Headings" list is right and shows "4". But when I go into the gallery itself, even though the numbers are right (4., 4.1, 4.1.1 etc) none of the headings are linked to the levels any more. I had to link headings 1-6 back to the levels again, and then my document numbers updated. I've tried it multiple times with the same result.

I thought, "ok, I'll just make my script do that as well!" so I added in ".LinkedStyle = "Heading 1" etc for all 6 levels and this did indeed leave me with a document with numbering updated, but still when I went into the gallery the styles weren't linked.

This feels like a bug to me: I've had this "de-linking" happen to me before seemingly at random. Perhaps there's a pattern of set of steps that leads to it, but I can't see them. Did it work right for you?