The master, at any time prior to the conclusion of the hearing, or the commission at any time prior to its determination, may allow or require amendments to the complaint or answer. The complaint may be amended to conform to the proofs or to set forth additional facts, whether occurring before or after the commencement of the hearing. In case such an amendment is made, the respondent shall be given reasonable time both to answer the amendment and to prepare and present his defense against the matters charged thereby.