Hi Jim,
I understand that you have verified your custom domain in Office 365, and now you would like to change your SharePoint site to www.customdomain.com.
You can achieve this goal by doing the following steps:
1. Log into MOP at https://portal.microsoftonline.com/ as Global administrator.
2. On the Admin page, in the left pane, under Management, click Domains.
3. Click Change address, and then step through the wizard.
For the detailed information, please refer to
http://onlinehelp.microsoft.com/en-us/office365-smallbusinesses/gg549209.aspx
Please feel free to post in this thread if there are anything unclear.
Thanks,
Allen Qiu