Typically organisations embarking on a Windows migration will be looking to utilise Active Directory (AD) as the principle source for people and profile information. From our experience AD is rarely up to date and complete, often missing key organisation structure, location and contact details.
Typically projects will use a federated data on a least worst approach which can cost time and effort and leaves the existing data gap.
To address this we would recommend establishing a User Data Strategy early on in the planning phase. This is to enable effective migration planning, end user communication and management information for the migration.
We would recommend automating the process for AD updates:
Ultimately at this point it's all about the users, and making sure the project has the data it needs to manage the migration effectively.