If you really want to let *everyone* know about this change, then have 'git push' with no parameters tell the user with the following message:
git push has changed the push command. It no longer defaults to ...
Do you want to keep the old default [y/N] N
Do you want to show this message again [y/N] N
Basically, have a warning that tells the user the change that is doing and ask the user if they want to keep the old default. If they do, then update the config to have git push stay the same.
Then ask the user if they want to see this message again, and if they say no, then update the config to not show the message.
I know now you are just trying to figure out if the change is good or not. But to make all users find out about this change, this intrusive message will do the trick. It may break a script here or there, but once the user says not to show the message, it will no longer be an issue.