Flying this AM from ORD to IAD. The gate agent announces: "we're going to try something new today; Families will preboard first, then active military, then those needing extra time, then Global Services and then 1K. We'll start in a few minutes." I was off to the side waiting to preboard as a GS. I walked up to the podium and told the GA that you can't just change the boarding order. It's posted above your head. Stick to the order United has created. He made a face and then called the order somewhat correctly but still called "families" before GS. I walked up and scanned my boarding pass and boarded. Maybe a bit DYKWIA but I don't think GAs should just make up the order.