How to change the owner of a story
For this you will need the Graphql id of the story (see How to get a Story) and the user you want to assign the story to.
graphqlmutation storyChangeOwner($input: StoryChangeOwnerInput!) {storyChangeOwner(input: $input) {clientMutationIdstory {iduser {id}}}}
variables{"input": {"id": "STORY_ID","owner": "OWNER_ID"}}
Expected result:
response{"data": {"storyTag": {"story": {"id": "STORY_ID","user": {"id": "OWNER_ID"}}}}}