change the 'changed' result of a task

tmp.yml

---
- hosts: localhost
  tasks:
    - name: create a tmp file
      lineinfile: dest=/tmp/head line=BODY create=yes

    # the task will be marked 'ok', because the output contains 'BODY', not 'HEAD'
    - name: cat head
      command: cat /tmp/head
      register: response
      changed_when: "'HEAD' in response.stdout"