Drupal 7: Change field type in the database

change_field_type.php

<?php
// Manually update the field type in the database to `list_text`.
  db_update('field_config')
    ->fields([
      'type' => 'list_text',
    ])
    ->condition('field_name', 'field_some_field_name', '=')
    ->execute();

  // Update the field type from 'int' to 'varchar' and update length.
  db_change_field('field_data_field_some_field_name', 'field_some_field_name_value', 'field_some_field_name_value', array(
    'type' => 'varchar',
    'length' => '255',
  ));
  db_change_field('field_revision_field_some_field_name', 'field_some_field_name_value', 'field_some_field_name_value', array(
    'type' => 'varchar',
    'length' => '255',
  ));

  // Update values in the database for this field since we've updated the
  // field type.
  $num_updated_yes = db_update('field_data_field_some_field_name')
    ->fields([
      'field_some_field_name_value' => 'Yes',
    ])
    ->condition('field_some_field_name_value', '1', '=')
    ->execute();
    
  $num_updated_no = db_update('field_data_field_some_field_name')
    ->fields([
      'field_some_field_name_value' => 'No',
    ])
    ->condition('field_some_field_name_value', '0', '=')
    ->execute();

  // Clear caches.
  field_cache_clear(TRUE);