I had a server at work that had been set up on Slackware a few years ago and left. It was Slackware 12.0. I wanted to get it up to date with all the security patches. I decided to use slackpkg to get it up to 13.0 in stages. The server was live, and in use, but not heavily in use.
slackpkg wasn't installed. I downloaded the appropriate slackpkg package from http://www.slackpkg.org/stable/ and went through them till I found the one that matched Slackware 12.0. I installed it with installpkg, aimed the mirror appropriately for Slackware 12.0 and let it rip. It updated all the security patches and got 12.0 right. I adjusted all the config files, and then rebooted.
It came up okay, so I aimed the mirror at Slackware 12.1 and let it rip. Adjusted the config files, rebooted, and it was still okay. I was feeling good at this stage. So I did it for Slackware 12.2 and then rebooted and it was still okay. Good thing this server is lightly used and wasn't interfering with customers.
So finally, it came to Slackware 13.0. I adjusted the mirror, let slackpkg do its thing, adjusted the config files, and rebooted once again. It came up okay. I checked it, everything was functioning, and it's all good.
I was very surprised. So now that server is completely up to date, all security issues up to date, and the upgrades were barely noticeable to customers. The only time they might have noticed anything was during the few minutes while the system was rebooting each of those times.
However, if something had gone wrong, they would have noticed because I would have had to install from scratch. I was ready, I had contingency plans, but they weren't needed. It was a good result.