[ansible] How to switch a user per task or set of tasks?

You can specify become_method to override the default method set in ansible.cfg (if any), and which can be set to one of sudo, su, pbrun, pfexec, doas, dzdo, ksu.

- name: I am confused
  command: 'whoami'
  become: true
  become_method: su
  become_user: some_user
  register: myidentity

- name: my secret identity
  debug:
    msg: '{{ myidentity.stdout }}'

Should display

TASK [my-task : my secret identity] ************************************************************
ok: [my_ansible_server] => {
    "msg": "some_user"
}