Changed permalink structure – old links no longer work

The easiest way for me to explain my problem is to create a guide on how to reproduce it. See below.

My question is: How do I add a rewrite rule to WordPress so that old URLs still work after updating the permalink structure?

Install WP & Update Permalinks
Go to Settings > Permalinks.
Select “Day and name” which displays format: /%year%/%monthnum%/%day%/%postname%/
Save.

Create a post for testing
Create a post called “Test”.
Permalink structure should be of format: /2015/09/21/test/ before publishing.
Publish.
Click “View post”.
Save URL.

Add actions/filters
Add this to functions.php:

function main_theme() {
  add_rewrite_tag('%MAIN_THEME%', '([^&/]+)');

  register_taxonomy(
    'main_theme',
    array('post'),
    array(
      'label' => 'Main Theme',
      'hierarchical' => true,
    )
  );
}
add_action('init', 'main_theme');

and

function main_theme_post_link($post_link, $post) {
  if (!empty($post) && $post->post_type == 'post') {
    $terms = wp_get_object_terms($post->ID, 'main_theme');
    if ($terms) {
      $theme = $terms[0]->slug;
      return str_replace('%MAIN_THEME%', $theme, $post_link);
    }
  }
  return $post_link;
}
add_filter('post_link', 'main_theme_post_link', 1, 3);
add_filter('post_type_link', 'main_theme_post_link', 1, 3);

Create category
Click on “Main Theme” located under “Posts”.
Create a new category called “Fruits”.

Update Permalinks
Go to Settings > Permalinks.
Select custom format: /%MAIN_THEME%/%postname%/
Save.

Create another post for testing
Create a post called “Test2”.
Permalink structure should be of format: /%MAIN_THEME%/test2/ before publishing.
Select “Fruits” from “Main Theme” in sidebar.
Publish.
Permalink should be updated and say: /fruits/test2/

Final test
Go to 2nd post by opening URL: /fruits/test2/
“Test2” should load correctly.

Problem!!
Now load 1st post by opening URL: /2015/09/21/test/
Page is redirected to URL: /%MAIN_THEME%/test/ and “Bad Request” is displayed.

How do I force WP to support old permalink structure as well as new structure?

I have tried adding the following to above init hooks without success:
add_rewrite_rule('^/([0-9]{4})/([0-9]{2})/([0-9]{2})/(.*)$', 'index.php?year=$matches[1]&monthnum=$matches[2]&day=$matches[3]&pagename=$matches[4]', 'top');