org-list-demote-modify-bullet

org-list-demote-modify-bullet is a variable defined in `org-list.el'.
Its value is

nil

Documentation:
Default bullet type installed when demoting an item.
This is an association list, for each bullet type, this alist will point
to the bullet that should be used when this item is demoted.
For example,

(setq org-list-demote-modify-bullet
'(("+" . "-") ("-" . "+") ("*" . "+")))

will make

+ Movies
+ Silence of the Lambs
+ My Cousin Vinny
+ Books
+ The Hunt for Red October
+ The Road to Omaha

into

+ Movies
- Silence of the Lambs
- My Cousin Vinny
+ Books
- The Hunt for Red October
- The Road to Omaha

You can customize this variable.