Good question, and I do not know the answer to it.
What type of file is the payload and what files does it contain? Is there a makefile in the payload to examine and from which you can derive what steps are taken to remove "old" files, and untar, relocate, and execute the payload?
What was the FTP source from where you got the system update files?
I notice that the directory structure of /PCBSD has a SystemUpdater Folder. Check out /PCBSD/SystemUpdater/system-updates. Inside that directory are two subdirectories: ./available and ./installed/1.5
In the ./installed/1.5 directory, I have the following files, all of which end in upd ("update")
- -rw-r--r-- 1 root wheel 228 Mar 15 12:52 031408print.upd
-rw-r--r-- 1 root wheel 228 Mar 20 17:33 03192008hal.upd
-rw-r--r-- 1 root wheel 223 Mar 21 09:57 kern-03202008.upd
-rw-r--r-- 1 root wheel 230 Mar 21 12:05 kern-03212008.upd
-rw-r--r-- 1 root wheel 225 Mar 29 17:13 patch-03292008.upd
-rw-r--r-- 1 root wheel 220 Mar 21 09:57 wifi-03202008.upd
Does this list match any of your files?
As an example, the wifi-03202008.upd contains the following lines:
I conclude that this .upd file is merely a link to the ftp site from which the upgrade can be downloaded. (i.e. ftp://ftp.pcbsd.org/pub/system-updates/i386/
I searched my disk, but I cannot find where my system put the upgrade files to get installed. Looks like the install cleaned up after itself.