This answer will explain the role your personal details play in domain registration at 123-reg, and why you should keep them up-to-date.
All domain names need to be registered to an individual or an organisation. It is very important that the information you provide is valid and accurate, as if there is a domain dispute later on, this information will be used to decide the legal owner of the domain name. If the details are out of date, misleading or otherwise incorrect then you will have no proof of ownership, and may be unable to transfer the domain.
Please note: If the details on your account are no longer correct, e.g. if you have moved house or the owner of the account has changed, you can change these in your 123-reg control panel.