Drush script to update changed field definitions for a price field, switched from integer to decimal. The script is run after the related code changes have been deployed, it updates the underlying database structures.

update-prices.php

<?php declare(strict_types=1);

/**
 * @file
 * Drush script to change the price fields from integer to decimal.
 */

use Drupal\Core\Database\Database;

// Check whether there are changes to field definitions.
$change_list = \Drupal::entityDefinitionUpdateManager()->getChangeSummary();
if (empty($change_list)) {
  \Drupal::messenger()
    ->addMessage(t('There are no changes to field definitions.'));
  return;
}

// Backup and clear the related tables.
$tables = ['node__field_prices', 'node_revision__field_prices'];
foreach ($tables as $table) {
  // Copy the table to a backup table.
  copy_table($table, $table . '_backup');
  // Truncate the original table.
  truncate_table($table);
}

// Make the changes to the field definitions. There's just one field.
$field_definition = \Drupal::entityDefinitionUpdateManager()
  ->getFieldStorageDefinition('field_prices', 'node');
\Drupal::entityDefinitionUpdateManager()
  ->updateFieldStorageDefinition($field_definition);

// Restore the data and drop the backup tables.
foreach ($tables as $table) {
  restore_table($table . '_backup', $table);
  drop_table($table . '_backup');
}

/**
 * Function to duplicate a table in the database.
 *
 * @param string $existing_table_name
 *  The name of the existing table.
 * @param string $new_table_name
 * The name of the new table.
 */
function copy_table(string $existing_table_name, string $new_table_name): void {
  // Get the database connection.
  $connection = Database::getConnection();

  // Build the query to copy the table structure and data.
  $query = "CREATE TABLE $new_table_name AS SELECT * FROM $existing_table_name";

  try {
    // Execute the query.
    $connection->query($query);
    \Drupal::messenger()
      ->addMessage(t('Table @new_table_name has been created successfully.', ['@new_table_name' => $new_table_name]));
  }
  catch (\Exception $e) {
    \Drupal::messenger()
      ->addError(t('Failed to create table: @message', ['@message' => $e->getMessage()]));
    exit(1);
  }
}

/**
 * Function to truncate a table in the database.
 *
 * @param string $table_name
 * The name of the table to truncate.
 */
function truncate_table(string $table_name): void {
  // Get the database connection.
  $connection = Database::getConnection();

  // Build the query to truncate the table.
  $query = "TRUNCATE TABLE $table_name";

  try {
    // Execute the query.
    $connection->query($query);
    \Drupal::messenger()
      ->addMessage(t('Table @table_name has been truncated successfully.', ['@table_name' => $table_name]));
  }
  catch (\Exception $e) {
    \Drupal::messenger()
      ->addError(t('Failed to truncate table: @message', ['@message' => $e->getMessage()]));
    exit(1);
  }
}

/**
 * Function to restore data from a backup table to the original table.
 *
 * @param string $backup_table_name
 * The name of the backup table.
 * @param string $original_table_name
 * The name of the original table.
 */
function restore_table(string $backup_table_name, string $original_table_name): void {
  // Get the database connection.
  $connection = Database::getConnection();

  // Build the query to restore the data from the backup table.
  $query = "INSERT INTO $original_table_name SELECT * FROM $backup_table_name";

  try {
    // Execute the query.
    $connection->query($query);
    \Drupal::messenger()
      ->addMessage(t('Data has been restored from @backup_table_name to @original_table_name.', [
        '@backup_table_name' => $backup_table_name,
        '@original_table_name' => $original_table_name,
      ]));
  }
  catch (\Exception $e) {
    \Drupal::messenger()
      ->addError(t('Failed to restore data: @message', ['@message' => $e->getMessage()]));
    exit(1);
  }
}

/**
 * Function to drop a table in the database.
 *
 * @param string $table_name
 * The name of the table to drop.
 */
function drop_table(string $table_name): void {
  // Get the database connection.
  $connection = Database::getConnection();

  // Build the query to drop the table.
  $query = "DROP TABLE $table_name";

  try {
    // Execute the query.
    $connection->query($query);
    \Drupal::messenger()
      ->addMessage(t('Table @table_name has been dropped successfully.', ['@table_name' => $table_name]));
  }
  catch (\Exception $e) {
    \Drupal::messenger()
      ->addError(t('Failed to drop table: @message', ['@message' => $e->getMessage()]));
    exit(1);
  }
}

This Gist is meant for fields defined programmatically, where the necessary changes have been made in code already.
To change the field settings for a field defined in the UI, the schema change must be run explicitly. Example:

$database = Database::getConnection();
if ($database->schema()
  ->fieldExists('node__field_prices', 'field_prices_value')) {
  $database->schema()
    ->changeField('node__field_prices', 'field_prices_value', 'field_prices_value', [
      'type' => 'numeric',
      'precision' => 10,
      'scale' => 2,
      'unsigned' => TRUE,
    ]);
  $database->schema()
    ->changeField('node_revision__field_prices', 'field_prices_value', 'field_prices_value', [
      'type' => 'numeric',
      'precision' => 10,
      'scale' => 2,
      'unsigned' => TRUE,
    ]);
}

Additionally, it might be necessary to adjust any keys using this field.