I did a text field from the Forms toolbar with bookmark 'example' in the body
of the document and I checked the 'Calculate on exit'. I put {example} on the
footer, but when I protect the document and change the text in a text field,
the {example} on the footer doesn't update automatically.

Here is the question: How can I change a text (text field or pure text) and
it updates automatically on the footer? The header has the same behaviour? I
ask this because the {example} on the body updates automatically (if the text
is a textfield and 'Calculate on exit' is checked).