Change Google Apps Primary Domain Name! How to change a primary domain name in Google Apps without losing you old mails?

To make sure that you don't lose your old mails while changing the primary domain of your Google Apps please follow the sequence of steps below.

The steps needed to start the domain name change.

Part 1—Add a secondary domain:
1. Navigate to the Admin console "Home" page at: https://admin.google.com/
2. If prompted to sign in, enter your Super Admin credentials
3. Click the "Domains" icon—it may be hidden under "More controls"
4. Select "Add/remove domains"
5. Click "ADD A DOMAIN OR DOMAIN ALIAS"
6. Select "Add another domain"
7. Enter a domain name for the secondary domain—you will later swap this with the primary domain.
8. Click "CONTINUE AND VERIFY DOMAIN OWNERSHIP"
9. Follow the on-screen instructions to verify domain ownership
10. After making the appropriate changes, click "VERIFY"
11. If successfully verified, a "Congratulations" message will be displayed
12. Click "Continue" to return to the "Domains" page
13. For more details on domain verification, please review the Help Center article at: https://support.google.com/a/answer/60216

Part 2—Create MX records for the secondary domain:
1. Stay on the "Domains" page
2. Look for the secondary domain that you added in Part 1
3. Under Status, click on "Set up Google MX records"
4. Follow the on-screen instructions to create MX records for your secondary domain
5. After making the appropriate changes, click "I have completed these steps"
6. For more details on creating MX records, please review the Help Center article at: https://support.google.com/a/answer/140034

Part 3—Enable API access:
1. Navigate back to the Admin console "Home" page at: https://admin.google.com/
2. If prompted to sign in, enter your Super Admin credentials
3. Click the "Security" icon—it may be hidden under "More controls"
4. Select "API reference"
5. If "Enable API access" is already selected, skip to the next part (below)
6. Click the checkbox adjacent to "Enable API access"
7. Click the "SAVE CHANGES" button

Part 4—Swap the primary domain via the Customer API:
1. Navigate to the Google Developers article at: https://developers.google.com/admin-sdk/directory/v1/reference/customers/update
2. Under "Try it!" select "Authorize requests using OAuth 2.0"
3. In the "Select OAuth 2.0 scopes" window, click "AUTHORIZE"
4. In the "customerKey" field, enter: "my_customer" (without quotes)
5. In the "Request body" field, click the "-- add a property --" drop down
6. Select "customerDomain"
7. Enter the secondary domain name that should be set as the primary domain
8. Click "EXECUTE"
9. If the primary domain has been renamed successfully, a "200 OK" message will displayed

Part 5—Confirm the primary domain changes:
1. Navigate back to the Admin console "Home" page at: https://admin.google.com/
2. If prompted to sign in, enter your Super Admin credentials
3. Click the "Domains" icon—it may be hidden under "More controls"
4. Select "Add/remove domains"
5. Confirm that the secondary domain is now the primary domain

Part 6—Rename users to new primary domain:
1. Navigate back to the Admin console "Home" page at: https://admin.google.com/
2. If prompted to sign in, enter your Super Admin credentials
3. Click the "Users" icon—it may be hidden under "More controls"
4. Click the user you wish to rename under the new primary domain
5. Click the "Account" link
6. Under "Basic Information", click the "Edit" link
7. Expand the drop down menu to the right of the @ symbol and select your new primary domain
8. Click "UPDATE USER"
9. Repeat these steps for all the users within your account that you wish to rename.