I found this when searching at linuxpackages.net.net
Building A Slackware Package.html

"I keep my scripts for several reasons, one for easy upgrade of
a package should I need to build a new version and so that I can
distribute them with my package. You need to be consistent with the
package, for example it must be installed the exact same way as you did
the first time so that upgradepkg will work with it and it will not
break any other programs."

"You need to be consistent with your filenames. This helps the
user use upgradepkg on your packages should you build a new version. If
not you can end up with stray junk all over the place since most will
not remove the old before installing the new. Which in some cases you
would not want to, due to conf files. You must use the standard naming
convention. The format is application-version-i386-1xxx.tgz"

I'm new to Slackware. I thought upgradepkg would just replace one package with another. How does upgradepkg work.

I want to upgrade a package I have installed with one I downloaded and compiled with a slackbuild script.

man upgradepkg