* PATH: use tools from the new system
* Merge new users/groups
* Interactively merge updated /etc files
  (borrow OpenBSD's sysmerge(8) or improve/simplify mergemaster(8))
* rcs(1) controlled /etc upgrade

[rcs-etc]
* rcs + vendor branch: to manage /etc and support 3-way merge to update
  the files
* etc-upgrade can be interrupted/paused and then resumed
* generate a detailed report for the upgrade, something like a git commit
* display the report and let user to make sure the upgrade is correct
* send the report as an email to root

* trap signal; clean up on error
* lock/status file to be safe ...