Amendments or supplements to any pleading will be allowed or refused in the discretion of the Administration if the case has not been assigned for hearing, otherwise in the discretion of the presiding officer designated to conduct the hearing; Provided, That after a prehearing conference has been held no amendment shall be allowed which would substantially broaden the issues, unless an opportunity is afforded all parties to answer such amended pleadings and to prepare for hearing upon the broadened issues. The presiding officer may direct a party to state its case more fully and in more detail by way of amendment. If a response to an amended pleading is necessary, it may be filed and served. Amendments or supplements allowed prior to hearing will be served in the same manner as the original pleading. Whenever by the regulations in this part a pleading is required to be subscribed, the amendment or supplement shall also be subscribed.