Larger organizations are likely to require weeks or months to complete a phased migration, and during your transition there will be some users on each of the two systems. Many organizations prefer that all users maintain access to an accurate enterprise directory (white pages, global address book), but accuracy will require regular synchronizations of the two servers' directories as employees are hired, transferred, terminated and so forth. Your Deployment Plan should therefore specify some mechanism for regularly synchronizing the two directories throughout your transition period.
Sun Professional Services can assist in addressing this issue. There are also several products available to perform directory synchronization.