I'm not sure the issue is related to the "Simpler" option, as the inablility to reassign a RealTrack seems to occur whether the "simpler" option is on or off, either at the track level or at the song level.

I guess I could just update directly to 2013 (which I have), skipping 2012.5 (which I also have), and hope that this "glitch" was resolved by a later build.

To Mac: the tracks are not frozen.