In the last big Slackware upgrade, I ran into a problem when some libraries changed and then wget would no longer work. I overcame this in ugly brute force method by setting a symbolic link.
I was browsing linuxquestions.org and found a whole set of discussions about the Friday 13th upgrade, and then found a handy hint that would have avoided the problems I had.
They recommend setting "DOWNLOAD_ALL=on" in /etc/slackpkg/slackpkg.conf. That will download all the files needed for the changes before installing them. This is good advice.