Complexity password Policy

Hi,

Currently we have a simple password policy which is not complex.

Now we are going to deploy Complexity password Policy.

If we deploy this Complexity password Policy today, does all the ( 3000 users ) user need to change their simple password to complexity password today .? or do they need to change when the password expiration date comes.?

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Our Setup:

Domain : Windows 2008 R2
Forest & Domain Level : Windows 2003

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Current Policy Setting :

Enforce password history      1  
Maximum password age         90 days
Minimum password age         0 days
Minimum password length     6 characters
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Planning do deploy following Complexity password Policy:

Enforce password history       3
Maximum password age        60 days
Minimum password age          0 day
Minimum password length        8
Password must meet complexity requirements    Enabled

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Any Ideas Please.

Thank You.

Regards,Ali