Using /fill x y z command_block 0 replace [block] doesn't work. Error is: Data parsing failed: Unable to locate name/value separator for string: [block].
Test:
1. Put stone below you
2. /fill ~ ~-1 ~ ~ ~-1 ~ command_block 0 replace stone
3. Data parsing failed: Unable to locate name/value separator for string: stone.

Compared to:

1. Put stone below you
2. /fill ~ ~-1 ~ ~ ~-1 ~ dirt 0 replace stone
3. 1 blocks filled