This article applies to 123-reg Shared Hosting packages.
The primary domain name associated with a 123-reg hosting package can not be changed once the hosting package has been purchased.
If you require another domain name to be hosted, you will need to use the Domain Mapping function available within the 123-reg Manage Hosting control panel.
Click the link below to find out how to setup domain mapping.
How do I map a domain name to my hosting?