How can I trigger rerender of an element in a preprocess function?
If I need to manipulate the text and/or display based on some PHP logic, I would do it using hook_node_view(). Implement this HOOK in you custom module. Use of this module will be like.

function MYMODULE_node_view($node, $view_mode = 'full', $langcode = NULL) {
    if( $view_mode == 'full' ){
        if( $node-&gt;type == 'YourContentType' ){
            $node-&gt;content['field_pb_doc_other_first_name'] = array(
                '#markup' =&gt; $node-&gt;content['field_pb_doc_other_first_name']['#markup'] . ' ' . mb_strtoupper($node-&gt;content['field_pb_doc_other_family_name']['#markup']);
        }
    }
}


My logic may be wrong, you need to check the $node variable inside this function for all the fields value.
The above code is just a sample code to demonstrate use of HOOK_node_view.