I just finished upgrading my Kubuntu-based desktop system using the instructions at the Kubuntu website.
It took about 35 minutes (ignoring download time) but I was able to upgrade within Kubuntu 18.04 to Kubuntu 18.10 and so far everything seems to work. I noticed that it adds back the original apps that came with Kubuntu that I had removed previously like Kmail and Firefox so I guess you basically get your customized distro/apps + whatever Kubuntu 18.10 comes with.Haven't noticed anything else different except for the browser integration icon in the tray that takes me to the Chrome page for adding Plasma integration.
Day two of using Kubuntu 18.10 and all is well. Using Oxygen theme.