How can I change the frequency of a scheduled event?

It looks for me that you are adding this event only when there is no such event ‘send_email_alerts_hook’ scheduled yet. Try something like this and let me know if it workded.

function shedule_email_alerts() {
    if ( !wp_next_scheduled( 'send_email_alerts_hook' ) ) {
        wp_schedule_event(time(), 'daily', 'send_email_alerts_hook');
    } else {
        wp_reschedule_event(time(), 'daily', 'send_email_alerts_hook');
    } 
}

The thing is that you will “rewrite” this event all the time, so it would be good to deactivate this function when it run first time.

The best solution would be to check how those jobs are gathered and check if this event is already added. If so then check if recurrence is different – if so reschedule. You can find this function in wp-includes/cron.php line ~63

function wp_schedule_event( $timestamp, $recurrence, $hook, $args = array()) {
    $crons = _get_cron_array();
    $schedules = wp_get_schedules();

    if ( !isset( $schedules[$recurrence] ) )
        return false;

    $event = (object) array( 'hook' => $hook, 'timestamp' => $timestamp, 'schedule' => $recurrence, 'args' => $args, 'interval' => $schedules[$recurrence]['interval'] );
    $event = apply_filters('schedule_event', $event);

    // A plugin disallowed this event
    if ( ! $event )
        return false;

    $key = md5(serialize($event->args));

    $crons[$event->timestamp][$event->hook][$key] = array( 'schedule' => $event->schedule, 'args' => $event->args, 'interval' => $event->interval );
    uksort( $crons, "strnatcasecmp" );
    _set_cron_array( $crons );
}

Good luck!