Make sure to tell your users before implementing this change what exactly a "complex password" looks like. if you don't do that, you'll end up with a lot of frustrated users and support calls.
Like another poster said, users can keep using their not-so-complex passwords without any problems until their password expires, and at that time they'll have to generate a new, complex password.
These are the rules you'll have to communicate:
? Is not based on the user?s account name.
? Contains at least six characters.
? Contains characters from three of the following four categories:
? Uppercase alphabet characters (A?Z)
? Lowercase alphabet characters (a?z)
? Arabic numerals (0?9)
? Nonalphanumeric characters (for example, !$#,%)