Simple script, change user IDs

I'd like a script which looks at 2 files, /etc/[login to view URL] and /etc/[login to view URL] and merges them with the system's current user and group lists. I believe this should be a relatively simple task for anyone experienced in linux administration. I believe that this can be done either in shell script or in perl, although I would be willing to accept other tools. If tools already exist to do this task, I would be willing to pay a small amount for a link. * The root user will not be touched. * Users who exist in [login to view URL] but not on the existing system should be created, and their home directories created. * Users who exist in [login to view URL] and on the existing system with the same UID should be left alone. * Users who exist in [login to view URL] and on the existing system with different UIDs should have their UID changed to match [login to view URL] and have ownership of all their files changed (except files on any filesystem mounted read-only) to use the new UID * Existing users on the system who are not mentioned in [login to view URL] should continue to exist. If their UID conflicts with a user mentioned in [login to view URL], they should be changed to the lowest non-conflicting UID over 500, and their files' ownership changed appropriately. * Groups and their GIDs will be handled in exactly the same way as users and their UIDs. * The list of group members for each group can be found by merging the lists of members from groups and [login to view URL] files - I believe that as /etc/groups contains a list of member usernames, this should not have any problems with the UID adjustments.