Hi Ian,
Before answering the question, I would like to confirm the situation:
1. Are you using custom domain as the email account suffix in Office 365?
2. Is the domain of the current email address in Outlook 2010 verified in Office 365?
If you want to change the email address suffix, the domain must be verified in Office 365 first. Please see Verify your domain and change name servers. And then you can change the email address in Office 365 Portal:
1. Login to Office 365 Portal (http://portal.microsoftonline.com).
2. Click Add users, reset passwords, and more under usres & groups.
3. Select a user and click the edit icon.
4. Change the user name and the domain under User name. The verified domains will be listed in the domain drop-down list.
5. Click save.
Best Regards,
Bruce Zhou