It's really hard to say what's involved without seeing the area and the light. You might be able to avoid replacing the siding if you put an outdoor rated cover plate over the box which the light was attached to and paint it the same color as the siding. It will be visible but will blend in. As far as the new location for the light, you will not be able to put it directly above the door (unless you are okay with a surface mount) since there is a wooden header beam right above the door. If you have room to put it somewhat higher up, that will probably work. However, some holes will probably be required to run the wires. I suggest you have an electrician come out to tell you your options and give you a price.