Move paragraphs from multiple limited-1 fields to a single components field with unlimited cardinality.

migrate-paragraphs.php

<?php

/** @var \Drupal\node\Entity\Node[] $landing_pages */
$landing_pages = \Drupal::entityTypeManager()
  ->getStorage('node')
  ->loadByProperties([
    'type' => 'landing_page'
  ]);
$old_paragraph_fields = [
  'field_tile_section',
  'field_leadership',
  'field_media_slider',
  'field_testimonial',
];
/** @var \Drupal\paragraphs\Entity\Paragraph[] $paragraphs_to_move */
foreach ($landing_pages as $landing_page) {
  $paragraphs_to_move = [];
  foreach ($old_paragraph_fields as $field) {
    if (!$landing_page->hasField($field) || $landing_page->{$field}->isEmpty()) {
      continue;
    }
    /** @var \Drupal\paragraphs\Entity\Paragraph[] $paragraphs */
    $paragraphs = $landing_page->{$field}->referencedEntities();
    foreach ($paragraphs as $paragraph) {
      $paragraphs_to_move[] = $paragraph;
    }
    $landing_page->{$field} = NULL;
  }
  foreach ($paragraphs_to_move as $paragraph) {
    $paragraph
      ->setParentEntity($landing_page, 'field_landing_components')
      ->save();
    $landing_page->field_landing_components[] = $paragraph;
  }
  $landing_page->save();
}