Migrating user accounts is not an easy task especially if you are a new admin. You need to make sure that file ownership, and password and group membership remains the same. The article does a good job of explaining the steps required to migrate from old Linux/UNIX server to news fresh installation. It’s a short and sweet tutorial and worth a read, especially for new admins to handle rescuing a dying server job. You can migrate users from old Linux server to new Linux sever with standard commands such as tar, awk, scp and others. This is also useful if you are using old Linux distribution such as Redhat 9 or Debian 2.x.

Source: Howto: Migrate user accounts and data from old Linux distribution to fresh installation