The notice of hearing or notice of opportunity to request a hearing may be amended once as a matter of course before an answer thereto is served, and each applicant or recipient may amend his answer once as a matter of course not later than 10 days before the date fixed for hearing but in no event later than 20 days from the date of service of his original answer. Otherwise a notice or answer may be amended only by leave of the hearing officer. An applicant or recipient shall file his answer to an amended notice within the time remaining for filing the answer to the original notice or within 10 days after service of the amended notice, whichever period may be the longer, unless the hearing officer otherwise orders.