Once an officer processes your I-485 (whenever that is, depending on your PD as well), you may receive a request for evidence that you continue to work in the same area of national interest that your petition is based on. At that time, you will probably have to provide a letter from your employer which hopefully you will have by then. Not all NIW filers get this kind of request (I didn't), but from what I've seen on this forum and others, most do.