Drupal 8 - Update a field type with data

update.php

  <?php
  
  $database = Drupal::database();
  $entity_type = 'paragraph';
  $field_name = 'field_value';
  $table = $entity_type . '__' . $field_name;

  $field_storage = FieldStorageConfig::loadByName($entity_type, $field_name);

  if (is_null($field_storage)) {
    return;
  }

  $rows = NULL;

  if ($database->schema()->tableExists($table)) {
    // The table data to restore after the update is completed.
    $rows = $database->select($table, 'n')
      ->fields('n')
      ->execute()
      ->fetchAll();
  }

  $new_fields = array();

  // Use existing field config for new field.
  foreach ($field_storage->getBundles() as $bundle => $label) {
    $field = FieldConfig::loadByName($entity_type, $bundle, $field_name);
    $new_field = $field->toArray();
    $new_field['field_type'] = 'decimal';
    $new_field['settings'] = array();

    $new_fields[] = $new_field;
  }

  // Deleting field storage which will also delete bundles(fields).
  $new_field_storage = $field_storage->toArray();
  $new_field_storage['type'] = 'decimal';

  $field_storage->delete();

  // Purge field data now to allow new field and field_storage with same name
  // to be created. You may need to increase batch size.
  field_purge_batch(10);

  // Create new field storage.
  $new_field_storage = FieldStorageConfig::create($new_field_storage);
  $new_field_storage->save();

  // Create new fields.
  foreach ($new_fields as $new_field) {
    $new_field = FieldConfig::create($new_field);
    $new_field->save();
  }

  // Restore existing data in the same table.
  if (!is_null($rows)) {
    foreach ($rows as $row) {
      $database->insert($table)
        ->fields((array)$row)
        ->execute();
    }
  }