A Custom Relationship Group is a construct in Financial Services Cloud that allows you to model complex and dynamic relationships between clients and accounts. You can use it to represent collaborative customer business operations that are not captured by standard household or account structures. Verified Reference: : Salesforce Financial Services Cloud Implementation Guide, page 67.

The following steps are required to migrate customer records from the Individual Model to Person Accounts in Financial Services Cloud:
Ensure Person Accounts is enabled on the org. You need to have Person Accounts enabled before you can convert Individual Model records to Person Accounts. You can enable Person Accounts by contacting Salesforce Support or by using the Enable Person Accounts option in Setup > Account Settings.
Configure your Person Account record types in the Individual Record Type Mapper. You need to map your existing Individual Model record types to the corresponding Person Account record types. You can do this by using the Individual Record Type Mapper option in Setup > Financial Services Settings.
Use a CSV file to map PersonRecordTypeId to the Person Account RecordTypeId and use Data Loader to update Client Records. You need to update the Client records with the correct PersonRecordTypeId value that matches the Person Account RecordTypeId value. You can do this by using a CSV file and Data Loader to perform a bulk update operation. Verified Reference: : Salesforce Help Article 1 : Salesforce Help Article 2 : Salesforce Help Article 3