If you can't warn us in advance of these changes, at least notify us afterwards, rather than leaving us wondering why people keep complaining that things aren't working with their sites.
I second this. Although mod_security has always been installed on my shared hosting server, I was very frustrated when the FTP default logins were disabled. Again, I think that a simple email wouldn't go amiss.
I do understand your concern regarding updating the clients regarding the changes made, but it is not possible to notify each and every client beforehand.
I don't understand how it should be any problem at all to email clients before/after making changes to their server. Afterall, you must have a system setup which will allow you to mass-email clients - no? If not - I suggest that one is setup. Even if it's an opt-in system - you can be sure that at least 2 people would appreciate it!
“A computer without a Microsoft operating system is like a dog without bricks tied to its head.”