Site.changePassword returns null when called using javascript remoting

My Use case is: As a CustomerCommunity user, I want to login and click on change_password link that redirects me to change_password page in AngularJs, which calles the changePassword method using javascript remoting, so that I can change my password. Issue: This method returns pageReference as null. this is my code: Visualforce Page:

    <apex:page controller="CustomCommunitiesLandingController" docType="html-5.0">
    <script>
     function  chgPwd() {
       var newPassword = document.getElementById('newPwd').value;
       var verifyNewPassword = document.getElementById('verPwd').value;
       var oldPassword = document.getElementById('oldPwd').value;
         Visualforce.remoting.Manager.invokeAction(
         '{!$RemoteAction.CustomCommunitiesLandingController.changePassword}',newPassword,verifyNewPassword,oldPassword,
         function(result, event) {
           alert("result");

         });
         } 
    </script>
     <div class="bootstrap"  >
    oldPwd <input type="text" id='oldPwd' />
     newpwd<input type="text" id='newPwd' />
     verpwd<input type="text" id='verPwd' />
     <h1 align="center">Click The Button</h1>
     <button onClick="chgPwd()" class="btn btn-lg btn-default btn-block">change password</button> 
     </div>
    </apex:page>

Controller Method:

    @remoteAction
    public static String changePassword(string newPassword, string verifyNewPassword, string oldPassword){
    pageReference pr;
    string success;
    pr = Site.changePassword(newPassword, verifyNewPassword, oldPassword);
    if(pr != null){
        success = 'true';
    }else {
        success = 'false';
    }
    return success;
}

Any help is greatly appreciated!

Thank you, Devayani