Mass editing contact's birthdays
With that many contacts whose records all need changing, I think I'd be inclined to let them do it themselves.  You could send them each an email with an editable profile including date of birth (required).  You could include a checksum to save them having to login and the profile form would then be pre-filled.  This could be a good time to keep their profiles up to date.

You dont say which CMS you are using but a useful link to token checksums is at https://wiki.civicrm.org/confluence/display/CRMDOC/Tokens.

Checksum for standard Profiles (edit mode): To send people to a profile use this path where N is the ID of the Profile you want to send them to:

Drupal: http://www.myorganization.org/civicrm/profile/edit?reset=1&amp;gid=N&amp;{contact.checksum}&amp;id={contact.contact_id}
Joomla!: http://www.myorganization.org/index.php?option=com_civicrm&amp;task=civicrm/profile/edit&amp;reset=1&amp;gid=N&amp;{contact.checksum}&amp;id={contact.contact_id}
WordPress: http://www.myorganization.org/?page=CiviCRM&amp;q=civicrm/profile/edit&amp;reset=1&amp;gid=N&amp;{contact.checksum}&amp;id={contact.contact_id} 


Hope this helps.