Changing password complexity on AD

Hi,

We are on Windows 2003 AD and at the moment we need to up the complexity of the passwords. We have around 400 users and the issue I have is I am worried once I cange this all users including service accounts will be informed all at the same time that there passords are not complex enough.

Will this come into effect the next time they are due to change passwords or from the moment I put the policy in place forcing password changes?

Thanks Tim