Because the multifield already has data, some subfield settings can no longer be changed.

to solve Error "Because the multifield already has data, some subfield settings can no longer be changed." in multifield drupal

you can change in multifield.moudle

if (multifield_type_has_data($form['#bundle'])) to 

if (multifield_type_has_data($form['#bundle1'])) 

this is easy way to remove this message