With a mixed cabin situation, it does not play well with travel waiver changes. I would call your travel agency and explain to them the circumstance, as well as the travel waiver in place. They *may* be able to reissue the ticket in business class for NRT-DEN, but most likely will need help from their United agency helpdesk. Have this resolved before traveling will reduce the potential headache the next day. If the agency person is not helpful, ask for a supervisor to assist.