I suppose the exact manner that an upgrade is performed is more of personal preference and ability for most Slack users. There can be configuration issues that may really demand a particular method.
With the new package name convention we have the additional choice of AutoPkg. But considering the size of a Slack distro, an excellent internet connection or a local ethernet ftp server is really required for the operation. Could be of a real value in a commercial site. I have considered attempting this myself in a test configuration.
In version 8.1 Pat offered an UPGRADE.TXT file that was dependent upon the filename convention for removing the old packages. If an upgrade to 8.1 to 9.0 is attempted this would not apply. But the rest of the process could remain the same.
Removing the old system and installing the new is probably the most often used method. The effort required here is dependent on how the system is configured and how many apps will need to be rebuilt.
When I consider my own system, a root RAID workstation, and the need for minimal downtime for critical apps with highest reliablity I use an indirect fresh install. I install fresh on a blank hard disk on a second computer and insure that is operable. Then I install a base primitive installation on a maintenance partition (2 GByte). From that base system I transfer the entire tested installation of the second computer to my primary root RAID partition (4 GByte). Minor config changes and it is back online. This is the only method I have had in the past to install on a root RAID system. My downtime has always been less than 4 hours. Pulling the CD's and recompiling my other apps as required is normally spread over a week or so. I also have separate RAID partitions for /root, /home, and a couple of other dedicated storage areas. This greatly reduces the amount of backup and restore time required to upgrade.
Thank you for asking the question. I know that I would be interested in how others upgrade as well, especially if configured for root RAID.