A demotion without cause? How is this legal?

I have been in a salaried position, with the title "Assistant to the Dept Director", with benefits, since 2006 with my current employer. My record is exemplary. I have no written disciplinary actions, no complaints, no personnel issues in all my 12 years. I have never taken sick leave or extended time away from work. I have been voted "best" employee by my colleagues, I have worked my personal life around the scheduling/teaching needs of my employer consistently for 12 years. My direct boss has never written me up for any reason. In January of 2018 our company changed payroll system programs. Last week (April 12th) the head personnel individual brought me in to let me know that because they changed payroll companies, I will no longer have a salaried position and will have to start working hourly. In addition, I will have to meet certain hourly requirements in order to retain the fulltime/salaried benefits I have had for the last 12 years. How is this legal? I have a job description that I've been working under since 2006 which outlines my title, my duties and my fulltime status. Our needs have no decreased - in fact I have spent the last month training a new staff member to join us. My workload hasn't diminished and there is no hiring freeze at our job. So what does a new payroll system for the company have to do with my job, my salary or my title? I have asked them pointedly if this is a demotion and I am still awaiting their reply. How is "because we do payroll differently now" a legal standing for cutting my income, removing my salary or removing my benefits that I've been working with for 12 years?