We are considering to change our domain name from:
reallylongandnotuserfriendly.example.com
to:
you.example.com
We have a Drupal 7 site. What are the consequences / fixes we need to be aware of, to not harm our CiviCRM installation?
Aside from any customizations, you should:
Check your civicrm.settings.php file to fix the base URL. (And double-check that your path hasn't changed).
Go to Administer - System Settings - Clear Caches and Update Paths and clear your caches and update your paths.
Confirm that the Resource URLs look right (they should be fine as a result of step 2).
Make sure you get a SSL certificate for your new domain name if you had one for your old domain name.
In addition to Andrew's answer, be aware that this may break your IPNs. See here for how to mitigate this.