Hi Eoin,
Regarding the statement mentioned above, you couldn't change the DNS records which automatically generates. When users change the Name Server records to point to Office 365, Office 365 will host the DNS record. On the contrary, custom domain hosts the DNS record.
After changing Name Sever records to point to Office 365:
If the website hosts on custom domain hosting provider, you need add a DNS record (A record) under DNS manager (Admin>Domains>Manage DNS). For detailed information about it, please try to add it via the following article:
Update DNS records to keep your website with your current hosting provider
http://onlinehelp.microsoft.com/en-us/office365-smallbusinesses/hh335298.aspx.
If the website hosts on Office 365, you don't need to add A record. After adding a custom domain, follow the steps to change the website address:
1. On the Admin page, in the left pane, under Management, click Domains.
2. Click Change address, and then step through the wizard.
Thanks,
Ida Qiu