Custom Excel Spreadsheet

We have a basic excel sheet we use to track job costs, we would like to have it updated to make it more robust and simple to change. One of the biggest problems we are having is wage changes. As you can see in the "Labor" sheet, in column O we have employee names and columns P & Q we have their hourly rate. During the course of a job an employees labor rate may change, however when we change the labor rate in column P it changes the labor rate for that employee for all work done prior to the wage change. We would like this workbook simplified if possible, perhaps with drop down menus or auto fill for employee names and a simple way to change the hourly rate without it effecting previous entries.