Edit system files

Hi,

I need to edit system property files, for instance in /System/Library/LaunchDaemons/ but it seems I'm not allowed to. I have tried using sudo and I also tried to enable the root account, but even as the root user I get the error "ftp.plist" E212: Can't open file for writing. (Using vim in this case. ftp.plist is just an example, it is the same for all files.)

Editing a file in a different location is no problem, but I still can't copy the file back to LaunchDaemons so it doesn't help.

I'm not even allowed to chmod files or the LaunchDaemons directory as the root user:

Library root# chmod 777 LaunchDaemons

chmod: Unable to change file mode on LaunchDaemons: Operation not permitted

Just to verify that I'm really running as root:

Library root# id

uid=0(root) gid=0(wheel) groups=...

ls -n shows that the owner of LaunchDaemons and all the files in it is root (0), so I find this really strange.

Searching the archives here I find several threads where folks are editing these files, so I must be doing something fundamentally wrong...

Are there recent changes in El Capitan that prevents editing these files?