According to MicroSoft’s Knowledge Base Article #292589, it’s a known issue and they have not posted any workaround:
When you sign in to the Web client computer in Microsoft Exchange Server 2003 or in Microsoft Exchange 2000 Server, you may receive a message that states that your password is about to expire in xx number of days. This message is not configurable, and the computer is hard-coded to display this message 14 days before your password expires.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the “Applies to” section.
The best (and easiest) solution is to use a 3rd party software solution that proactively notifies users of upcoming password expiration.
The best software tool that I have found is Password Reminder PRO from SysOp Tools.
It’s low cost, easy to setup / deploy and can be used free for 60 days. It also allows you to test in your live domain without disturbing users. Highly recommended, definitely makes up for the noted issue.