The currently suggested syntax is:
/fill <x1> <y1> <z1> <x2> <y2> <z2> <TileName> [dataValue] [oldBlockHandling] [dataTag]
However based on MC-61864, it should rather look like one of these:
/fill <x1> <y1> <z1> <x2> <y2> <z2> <TileName> [dataValue] [[oldBlockHandling] [dataTag]|[replace <TileName> [dataValue]]]
/fill <x1> <y1> <z1> <x2> <y2> <z2> <TileName> [dataValue] [oldBlockHandling] [dataTag] OR
/fill <x1> <y1> <z1> <x2> <y2> <z2> <TileName> [dataValue] replace <TileName> [dataValue]
Confirmed.