The problem with your question is that it assumes that the par of a course can be changed. Of course, it can, but if par is to have any real meaning, changing par assumes that the course is also changed and not just the par designation. There is a correct way of setting par. If you take the position that par is subjective, then to you, "par" and "difficulty" should be essentially meaningless. However, if there is a correct way to set par, and their is (as well as a correct person to set it), then "difficulty" can also easily and meaningfully be defined in terms of actual score in relation to par.