Hi laufrie,
You should remove the domain from the previous Office 365. If you encountered some errors when you remove domain, please refer to the link below to troubleshoot the issue :
Remove Office 365 Domain:
http://www.configureoffice365.com/remove-office-365-domain/
After you have removed the domain from the previous Office 365, you can add the domain in the new Office 365. About how to verify domain in Office 365, please refer to the link below :
For Office 365 for small business:
Add your domain to Office 365 http://onlinehelp.microsoft.com/en-us/office365-smallbusinesses/ff637561.aspx
Verify a domain at a domain name registrar http://onlinehelp.microsoft.com/en-us/office365-smallbusinesses/gg584174.aspx
Change name server records at a domain registrar http://onlinehelp.microsoft.com/en-us/office365-smallbusinesses/gg549203.aspx
For Office 365 for enterprise:
Add your domain to Office 365 : http://onlinehelp.microsoft.com/en-us/office365-enterprises/ff637620.aspx
Verify a domain at any domain name registrar: http://onlinehelp.microsoft.com/en-us/office365-enterprises/gg584188.aspx
Edit DNS records for your Office 365 services : http://onlinehelp.microsoft.com/en-us/office365-enterprises/ff637620.aspx
Thanks,
Linda Wang