I don't know about upgradepkg, but here is the method that will surely work (It worked for me many times ): you should use pkgtoll to remove all libraries related to KDE, as well as the KDE itself and the XFree. So everithing like Qt, KDevelop and other apps that are somehow related to KDE and came with the Slack9 destribution should be removed using the root console. Then use some ftp client(mc will do just fine) to connect to some Slackware mirror site (for example ftp.heanet.ie) and go to the current directory of Slackware destribution. Download and install the KDE and it's libraries, apps and everithing you need from that FTP. I installed KDE 3.2 without any problems this way.
Note, that for some reason the newly installed KDE doesn't start properly right away. You should reboot the system and then try again(you should type startx before reboot, after this fails reboot and everithing will start OK). And you also should remove the xinitrc and the KDE folder from your user directory(infact everithing related to KDE).
Hope this works for you.