I remember hearing an opinion somewhere that nusach Ashkenaz and nusach Sefard (but not real nusach Sfaradi/Edot Hamizrach!) are both basically the same nusach, since they are structurally very similar. I think this would mean that a chazan in an Ashkenazic kehillah could use either nusach without worrying about lo tisgodedu. Is this a real opinion, and if so, who brought it down?