Open OnCall. Right-click on an assignment and select Shift Times.

The shift-times toolbox has a solid down-pointing black arrow that sets new default hours for the service. First customize the times on the day the times change. Then click the down-pointing black arrow to make the new times the default from that day forward.

A small gray triangle in the upper-left of a cell denotes default hours that differ from the times a shift is currently set to span. The gray triangles display for a few weeks before the times change. If you move your schedule ahead and change the default hours a 2nd time, the day the hours change from one old time to the next will have a white tab in the upper left.