As far as I know, Swaret only installs pre-made Slackware packages. If you want to upgrade to something newer than Slackware current (9.1 ATM) you have to compile and install yourself.
The problem is that if you do that, you can't use Swaret or PKGTOOL to upgrade them.
What you want to do is download and compile the version of KDE that you want but follow one of the many Slackware Package Creation Guides on the net and make a Slackware package and use PKGTOOL to install it.
You might also ckeck
www.LinuxPackages.net and see if someone hasn't already done the work for you.