Enabling Password Complexity

Hi All,

We have a password policy in our company that's doesn't have password complexity enabled. We want to enable it but we are worried as we have 400 users. IF we enable it will it enforce the policy on all the users immediately and will force them to change the password ??
Another thing we are concerned is when we enable the Password Complexity option will it give user the option to choose 3 out of 4 password complexity options or the user has to have all 4.

I know, the option explains itself but we don't want to risk on 400 users and start getting tickets on this.
If someone who has experience on doing this, please help me figure this piece out.

Thanks