Bit of a note for my future self if I ever do a version update of the AMAZING Gran Turismo 4 Spec II.
First, don’t bother upgrading - it probably won’t work. Maybe for other people, but I convert PS2 *.iso’s to *.chd files. Converting that back to patch doesn’t like it.
Just go with a full patch. Look for the Gran Turismo 4 Online Public Beta (US) - [SCUS-97436] image on a place like archive.org.
Then remember that xdelta is installed in macos via homebrew and called with xdelta3, as that’s the latest version.
$ xdelta3 -d -s [source file.iso] [GT4 patch.xdelta] [output file.iso]
$ chdman createcd -i [output file.iso] -o [Gran Turismo 4 Spec II.chd]