I have an FP45 with sound nscale and had no trouble changing the address to a 4 digit address using and MRC Prodigy Advance2 on the programming track.
Though I'm not sure,I believe you have an MRC decoder (Athearn's steamers do).I've programmed both my Challenger and Big Boy on the main without any problem.Read your instructions in your Digitrax manual and you'll likely have to toggle modes until one works.I don't remember wich did in my case.