Take a lesson from the theater world: Practice makes perfect. Why not perform a dress rehearsal on a different machine before you perform the upgrade? Doing so might reveal unexpected roadblocks, as well as give you an indication of how long the process will take. A dress rehearsal requires a lot of resources. However, if you are about to perform the first upgrade of many, this can be a valuable tool to estimate the time the upgrades will require.
An absolutely complete dress rehearsal results in a new machine that can leaving the system partitions as generic as possible. Such additions to the system can be documented in a CHANGELOG file. Most changes will be in /etc, which is small enough to be copied before any upgrades begin and used as a reference. That is preferable to the laborious process of restoring files from tape.
In a dataless UNIX environment—all machines have an OS local but otherwise get all data from a server—usually only /var needs to be preserved between upgrades and then only the crontabs and at jobs, the mail spool, and, for systems such as Solaris, the calendar manager files. A version control system, such as RCS, is good for tracking changes to configuration files.