Changing CiviCRM Site Key in the civicrm.settings.php file
Changing the site key will break a few things. How many of these are important is up to you, but at least:
- every rest interface that connects to the site to read or write (you might not have any)
- any hash (eg. link to update preferences). this might not be an issue if the domain name changes too
- anything that is saved encrypted (eg. the password for the smtp connection).

search the code base for CIVICRM_SITE_KEY to have a complete list.

This being said, the easiest is probably simply to change and wait for a while to see if it breaks any of the features YOU use.

The value of the key itself doesn't matter, as long as it's hard to guess and long enough. The setup process generates it that way:

md5(uniqid('', TRUE) . $params['baseURL'])


so a random hash of 32 characters