Changing footway=both/right/left/none to sidewalk=both/right/left/none
I suggest to set footway=both/right/left/none deprecated and retag it to sidewalk=both/right/left/none. I can do some of this work with XAPI and JOSM. The database will get clearer. The technique of tagging the sidewalks as additional tag is not involved, nor changed.
According to Automated Edits code of conduct I start this discussion also in the OSM forum.