Aaaannd... My upgrade is complete. I used the removepkg command to remove infozip, and did the wget and installpkg commands to manually re-install it. I then re-ran the installplg command and all is up and working! Here is the wget command for infozip:
wget http://slackware.cs.utah.edu/pub/slackware/slackware-13.1/slackware/a/infozip-6.0-i486-1.txz -O /boot/extra/infozip-6.0-i486-1.txz
Thanks
buzz