This won't be much help, but here is something else to think about. Coach2709 has replied on how to lower a local high spot, but if you are thinking about changing the overall slope of the field, or anything involving moving large amounts of earth, then you probably would need to worry about drainage of the field. A field that is nice and flat and the same height everywhere has no obvious path for rain or over-irrigation to escape. Here in California, many fields have been converted to artificial turf (not the carpet type, but the long plastic fibers held down by little black rubber balls). These fields are very level, and I'm told that providing a drainage network is a much bigger expense than the grading. I think the same considerations would apply to a grass field if it is made very level.