I feel as though I am missing something about your question.
In a task report, I would select everything and then click on the edit button to edit in bulk (there's one right under the report title).
From there you click on the Assignments bookmark to go to that section of the form.
When you click on the Assignments checkmark it should give you two sections: one for you to add a person and one for you to remove a person.
You would put Isabella in the second field and Anna in the first field, or so I would think. But is this the part that isn't working? If so, what's it doing? (is it possible Anna's already assigned to some of the things Isabella is assigned to?)