Er, those changes will likely be overwritten every time you run a Software Update that comes with a newer version. Also, you don't have to do that. If all you want is add the disabled key, use launchctl unload -w.
Code:
unload [-w] paths ...
Unload the specified configuration files or directories of con-
figuration files.
-w Add the disabled key and write the configuration files
back out to disk.
Playing around with Apple's plists manually is a bad idea.