Update the column structure for a field type.

update_field_definition.php

<?php

/**
 * Converts the value column from integer to float.
 */
function mymodule_update_70XX() {
  $spec = array(
    'type' => 'float',
    'not null' => FALSE,
  );  // DB Column definition.
  $field_name = 'your_field_name';
  $field_column = 'your_column_name';

  $fields = array_filter(field_info_fields(), function($item) use ($field_name) {
    return $item['type'] == $field_name;
  });

  foreach ($fields as $field_definition) {
    $data_table = 'field_data_' . $field_definition['field_name'];
    $revision_table = 'field_revision_' . $field_definition['field_name'];
    $field = $field_definition['field_name'] . '_' . $field_column;

    db_change_field($data_table, $field, $field, $spec);
    db_change_field($revision_table, $field, $field, $spec);
  }
  
  return t('Field structure updated.');
}