Code snippet to merge several paragraphs fields into a single field.

merge_landing_paragraphs.php

<?php

$nodeManager = \Drupal::entityTypeManager()->getStorage('node');

$nodes = \Drupal::entityTypeManager()
  ->getStorage('node')
  ->loadByProperties([
    'type' => 'landing'
]);

$fields = [
  'field_landing_top',
  'field_landing_highlighted',
  'field_landing_middle',
  'field_landing_bottom',
];

foreach ($nodes as $node) {
  echo 'Updating node ' . $node->id() . ' - ' . $node->label() . PHP_EOL;
  foreach ($fields as $field) {
    echo 'Syncing field ' . $field . PHP_EOL;
    if ($node->hasField($field)) {
      $paragraphs = $node->get($field)->referencedEntities();
      if (!empty($paragraphs)) {
        foreach ($paragraphs as $paragraph) {
          $node->field_components[] = [
            'target_id' => $paragraph->id(),
            'target_revision_id' => $paragraph->getRevisionId(),
          ];
          echo $paragraph->id() . ' - ' . $paragraph->getType() . PHP_EOL;
        }
      }
      // Unset paragraph data
      $node->$field = [];
    }
  }

  $node->save();
}

// use with Drupal Console
// copy file to `console/snippet/` directory
// drupal ahoy drupal snippet --file=console/snippet/merge_landing_paragraphs.php