/fillreplace is not under the help menu and wont autofill

There is no command fillreplace, but fill does autocomplete it's name and parameters, see screenshot.

http://www.minecraftforum.net/

NOTABLE MAPMAKER ADDITIONS:

New command /particle to show particle effects in the world. It can also be relative to entities. Example: /particle lava ~ ~1 ~ 1 1 1 0.5 2 @e[type=Chicken]
Added the ability to filter block types in the /fill command, but only if it’s used to fill in replace mode with “normal” blocks. It does not work when filling with block entities. Syntax: /fillreplace [filterBlockName] [filterData]
Fixed most of the remaining issues with the /clone command, for example chest orientation.

never mind i figured it out

replaces all stone with dirt inside area
fill -333 ~-1 274 -330 ~1 271 minecraft:dirt 0 replace minecraft:stone

Syntax: /fill [x] [y] [z] [x] [y] [z] [BlockName] [Data] replace [filterBlockName] [filterData]

That was a copy-and-paste error of MCF.
First hand information was https://mojang.com/2014/01/minecraft-snapshot-14w04a/

Notable additions for mapmakers:
....
Added the ability to filter block types in the /fill command, but only if it’s used to fill in replace mode with “normal” blocks. It does not work when filling with block entities. Syntax: /fill replace [filterBlockName] [filterData]
...