I want to change my domain name from reallylongandnotuserfriendly.example.com to you.example.com.
Is there a setting that could expose these change to the public, without actually changing the real domain name?
You could create a Drupal alias as follows:
Add the following line
$sites['you.example.com'] = 'name_of_the_directory_where_your_settings.php_is';
In your case, the settings.php file is likely to be inside sites in a directory called reallylongandnotuserfriendly.example.com, which means the line would be:
$site['you.example.com'] = 'reallylongandnotuserfriendly.example.com';
Just be sure that you configure your server to point you.example.com to your base Drupal installation.
$base_url
in that file$base_url
change the file permissions for that file, since by default it is a read-only file.