[CentOS] Migrating users to new server

An age-old problem and there are some simple awk/grep-based scripts to 
handle the easy cases. Are there any more sophisticated tools that can 
extract all non-system users and groups and then merge the result into the 
password/group/shadow/gshadow files of a new server? I don't need home 
directories or mbox inboxes created because I'll rsync those over once I've 
created the users.

Tricky problems include merging entries for existing users and groups, and 
renumbering if one is moving from a 500-based system to a 1000-based 
system. (I did the latter long ago so don't need it this time.)

Red Hat's page with some simple commands to handle the easy cases:

<https://access.redhat.com/solutions/179753>