I followed these steps and it worked correctly:
1. Removed an existing book from my challenge.
2. Added
{'Books Purchased in 2024': {'#admintags' : 'readinggoal
urchases2024'}} to my 'add to challenges' config.
3. Added
readinggoal
urchases2024 to the #admintags column in the book I'd previously removed.
When I checked, the book had been re-added to the challenge.
EDIT: Ignore the smileys, lol.