change_field_type.php

<?php 
/*
 * Change type for existing Field. Drupal.
 */

$field = 'field_example';

db_change_field('field_data_' . $field, $field . '_value', $field . '_value', array(
  'type' => 'numeric',
  'precision' => 10,
  'scale' => 2,
  'not null' => FALSE,
));

db_change_field('field_revision_' . $field, $field . '_value', $field . '_value', array(
  'type' => 'numeric',
  'precision' => 10,
  'scale' => 2,
  'not null' => FALSE,
));

$s = db_query("SELECT * FROM {field_config} where field_name = :name", array(':name' => $field))->fetchObject();
$data = unserialize($s->data);

$data['settings'] = array(
  'precision' => '10',
  'scale' => '2',
  'decimal_separator' => '.',
);
$data = serialize($data);

db_update('field_config')
  ->fields(array(
    'data' => serialize($data),
    'type' => 'number_decimal',
  ))
  ->condition('field_name', $field)
  ->execute();