Moving Wordpress Custom Field Value into Default Editor/Excerpt

I am in the process of migrating posts between an old and a new website. ACF was used on the old website and when I import the posts into the new website it creates the custom fields "banner_description" and "main_description".

I would like to do away with these custom fields and have the values from each be put into the default Excerpt and Editor text boxes.

banner_description -> Excerpt

main_descriptoin -> Default WYSIWYG Editor

How would I go about doing this?

Utilize the wp_update_post() function to update the content and excerpt of each post you want. Pull in the ACF data you need with the get_field() ACF function so you can set the content and excerpt accordingly. You can call the update function via the init action.

Note: Recommended to only add and run this init action once and then remove the function so it won't be called every time a page loads.

function update_posts_content_excerpt() {
  $args = array(
    'post_type'      => 'post',
    'posts_per_page' => -1,
    'post_status'    => 'publish'
  );
  $posts_array = get_posts( $args );

  // Loop through those posts 1 by 1
  foreach ($posts_array as $post) {
    $post_id = $post->ID;

    // Get current value of the ACF fields
    $banner_description = get_field("banner_description", $post_id);
    $main_description = get_field("main_description", $post_id);

    // Create array of data to update for the post
    $post_update_data = array(
      'ID'           => $post_id,
      'post_excerpt' => $banner_description,
      'post_content' => $main_description
    );
    // Update the post into the database
    wp_update_post( $post_update_data );
  }
}
add_action('init', 'update_posts_content_excerpt');