Although it is possible to upgrade, (I've been upgrading continuously since Slackware 3.5) I highly recommend a clean install if possible. It is much, much easier to copy your files you want to save elsewhere, install clean, and then copy them back and/or editing the new config files to do what your old ones did. The pkgtools includes upgradepkg, installpkg, and removepkg... see man pages and the Slackware Book on these. The upgradepkg tool is, IMHO, best used to upgrade packages within a major release, but to jump from 8.1 to current (9 beta 1), install clean. If you do the upgradepkg route you have to be pretty careful to upgrade certain packages first, and in a specific order... and even then might have to manually fix some sym-links, etc. Just install clean. Then, it's easy.
I actually recommend current as it is right now, but it is beta... your mileage may vary. I've had good luck with it... installing it is the same as for 8.1 if you can burn a cd... no special wizardry needed. CDRecord works good for me. There are instructions on making a bootable install CD with CDRecord in the isolinux/ directory. Note: you have to exclude either Gnome or KDE, on your install CD to fit everthing else on an install CD as explained in the ISOLINUX/README.
ftp.slackware.com/pub/slackware/slackware-current has the most current of current... but it is dog-slow... I usually get about 7Kbps at best.
There are several mirror sites that are much, much faster but may be from several days to several weeks behind current at slackware.com.
If you use ftp (and I just happen to really like Gftp), you can download -current from a mirror, and then just download the few files that have changed in the last few days, if any, from slackware.com using the Gftp <Tools> <Compare Windows> to flag the files that are different. (This is just how I do it. Others may recommend other methods).
A cool list of mirrors are listed at http://wildwizard.kicks-ass.org/ as well as at slackware.com.
What the heck. give it a whirl! You'll then be all set for when Slackware 9 is officially released.