If you aren't using Drupal's multisite system (that is, the site information is being put into sites/default instead of sites/example.org or sites/example.com), this is really easy: do nothing. There is no step 2. Drupal will handle any domain thrown at it: just use your web server's name-based handling to determine which domains to accept.
If you are using Drupal's multisite system, it's a little harder:
- Rename the sites folder (so from sites/example.com to sites/example.org).
- Drupal's files table hardcodes the files directory it uses, so you're going to need to change all the paths in it. Run
UPDATE files SET filepath = REPLACE(filepath,'example.org','example.com') on the database.
- The other place to change the path is under Site Configuration -> File system.
- The final place you're going to need to change paths is going to be on the settings page for your theme if you decided to use a custom logo or favicon.
- Once you've changed all the paths, rebuild the cache under Site Configuration -> Performance.
This should cover most cases. Check with the third-party modules you're using to see if they also hardcode filepaths. If the modules are using the API correctly, they should've been handled in steps 2 and 3.
Because of all the hardcoded file paths, I highly recommend not using multisite unless you really need to. If you are only running one site in a Drupal install, just stick the site in sites/default.