Postfix cannot be updated by the current Slackbuild because Postfix uses a nonstandard internal update mechanism. The Slackbuild creates ".new" configuration files just like all Slackbuilds, but the internal postfix upgrade, which changes file permissions, etc. requires that the configuration file supplied in the package be actually installed before it can run successfully. With the old configuration files, the internal upgrade fails, and you wind up with a busted install.
A workaround is to rename all the configuration files in /etc/postfix to /etc/postfix/<name>.bak, then run the Slackbuild. Your customizations can then be hand-merged from the".bak" files into the new configuration files. (This would have to be done under any circumstances). The Slackbuild could be modified to do the renaming, but this would be inconsistent with virtually every other Slackbuild, and would be very confusing. A really clean solution would need to come from the Postfix developers.