I think I can now answer my own question, after studying this article
https://blog.graphisoftus.com/tips-and-tricks/introduction-to-archicad-markers by Jared Banks and Nalthan Hildebrant's Skewed template for AC21.
Copy the elevation viewset from the sketch design subset into the CD subset. The ID on the views can be changed in the new set without affecting the marker. If the markers on the SK plan are set to display the ID for the first placed drawing of the viewpoint, they will reference the sketch design elevation drawing on the sketch design elevation layout. When the sketch design stage is complete and the CD references are required, drag the whole sketch design layout subset to the bottom of the tree, or at least below the CD subset. The marker on the plan will then update to reference the alternative ID on the CD documentation and CD elevation layout instead of the SK elevation layout.
Possibly this is news to no-one, but it's a very happy revelation to me. Would be glad to hear if anyone has any better ideas, or to hear about what other workarounds are used.
AC26 Australia, Windows 10