You might go to Harley Nolans forum on this site and ask about the value of that sistema before you change anything. If it won't change the collectors value, I'd antique it in a heartbeat.
As for corrosion resistance, the plum brown seems to hold up very well. Corrosion really hasn't been a problem with any of my guns though because I always clean them right after I'm done shooting. But, when I had the gun "in the white" I did notice some staining on the metal after shooting that would not come off with regular cleaning and I'd take some 0000 steel wool to it to re-shine the metal. No worries about that kind of thing with this finish though.
As for corrosion resistance, the plum brown seems to hold up very well. Corrosion really hasn't been a problem with any of my guns though because I always clean them right after I'm done shooting. But, when I had the gun "in the white" I did notice some staining on the metal after shooting that would not come off with regular cleaning and I'd take some 0000 steel wool to it to re-shine the metal. No worries about that kind of thing with this finish though.