Reduction in PTO to add equal amount of Sick leave. I had negotiated my PTO when hired. How to argue to retain the agreed upon PTO?

My company announced that they are reducing PTO by 40 hours to introduce 40 hours of sick leave. When I originally signed my contract I negotiated and increased amount of PTO. They believe they are honoring the agreement by reducing my PTO to give me sick time. As there is no substantiation or documentation required to use sick time, they are saying it's not a reduction of PTO. I disagree and I do not accept this. I'm going back and forth with HR. Am I out of line?