Changing a primary domain name in Google Apps has never been easier but here are things you should consider before making the change.
The first thing to consider before changing your Google Apps Primary Domain is to find out if you are able to. Google has a migration path but some accounts are not able to use it. You can not migrate your Apps Primary Domain if you:
Google Apps provides configurations that will easily allow you to use domains other than your primary domain. You can add additional domains into your set of managed domains. There are limitations involved with multiple domains but they are fairly manageable.
One option is a domain alias. Domain aliases allow your organization to have emails addresses in different domains that will deliver email to the primary domain user. This allows users to have email addresses at @old_domain.com & @new_domain.com, and both will deliver to @old_domain.com. A second option is the separate domain. This is best suited to user accounts at different domains that should share the same Google services. Alternatively, setting users up in a separate domain can be an efficient way to transition users to a new domain.
If multiple domains won't work, then it is time to plan for renaming your primary domain. Renaming your Apps Primary Domain won't automatically rename users. Tools that store user connections won't automatically update. It is best to assume that any service identifying or pointing to specific users will have to be manually updated.
Email is the most commonly used of any Google application and it will need the most attention. MX (Mail Exchange) records will need to be created pointing to the new domain. MX records inform the Google Apps servers where to send incoming emails.
Changing the domain will not update the primary email address for any of the domain's users. You will need to rename all users to the new domain. After you rename a user their old address will become an email alias for their new address to avoid losing emails. All renamed users will need to create new Gmail signatures for their new addresses. Read receipts from emails sent by the old email address will not be forwarded to the new address.
Some data in certain services will still show the old domain address. Contact records for your users will show the old email address as well as the new one. The old email address will have an "Obsolete" tag. A user's old address will remain their email address in their friend's circles. Calendar events created before the rename will still show the old email address.
Changing your Google Apps Primary Domain is sometimes necessary. Hopefully a domain alias or a secondary domain will accomplish what is needed. If additional domains won't work then a Google Apps Primary Domain rename will be necessary. Performing the domain rename can cause short term user impact so make sure you have a plan, and your people are aware of what changes to make to their accounts. Regardless of the direction you choose, Google has the tools to help make your experience as smooth as possible.