(1) Any pleading or document may be amended prior to notice of the hearing. After notice of a hearing is issued, motion for leave to amend any pleading or document may be filed with the commission and may be authorized in the discretion of the commission or the hearing examiner. Any amendments filed shall contain a certificate of service upon all known interested parties. Post-notice amendments to any pleading or document shall not unduly broaden the scope of the issues originally filed with the commission, unless the commission shall in its discretion allow such amendments. If a post-notice amendment is approved, the commission shall afford the parties notice of the approval and adequate opportunity to prepare for hearing.