how to change the value of the form in the preprocess functions or get the value of the function preprocess
You pass extra arguments when building the form using the drupal_get_form.

For example:

drupal_get_form('comment_email_form', 'default_value');

function comment_email_form($form, &amp;$form_state, $default_value)
{
    // $default_value is here the string 'default_value' value passed as extra argument
    // ... rest of your form
}