/*hide the original password protected message*/
.post-password-form p:nth-of-type(1) {
    visibility: hidden;
    position: relative;
}

/*new password protected message*/
.post-password-form p:nth-of-type(1):after {
    content: 'NEW PASSWORD PROTECTED MESSAGE GOES HERE';
    visibility: visible;
    position: absolute;
    left: 0;
}