hook_theme_registry_alter issue

/*
Below are the functions and places i cannot get my head around
i try to override a functionality in (1) whith a theme_preprocess function (3) but..
there is a registry_alter function (2) and my overrides in the template.php are not kicking in..
what can i do?
*/


/*
* 1: function in date.module
* (modules\calendar\theme\theme.inc)
*/

function template_preprocess_calendar_datebox(&$vars) {
// do this and that
}

/*
* 2: function in calendar_tooltips.module
* (modules\calendar_tooltips\calendar_tooltips.module)
*/

function calendar_tooltips_theme_registry_alter(&$theme_registry) {
  if (!empty($theme_registry['preprocess_calendar_datebox'])) {
    $theme_registry['preprocess_calendar_datebox']['function'] =
      'calendar_tooltips_preprocess_calendar_datebox';
  }
// (..other stuff)
}

/*
* 3: function in a mytheme
* themes\mytheme\template.php
*/


function mytheme_preprocess_calendar_datebox(&$vars) {
// do stuff
}