Miss(es) Hightree might also just be reporting old information.
Did you contact HGPCS to see which number was currently assigned to your account? If it is your number, then I'd just disregard Miss(es) Hightree's note.
If it is still your Mother's, then I'd make the change though your personal profile on hyatt.com (I think it shows the number, or at least the last four digits) to make sure it sticks.