Both commands should do "the same" but if you put command_block in the left place it will expect a dataTag where the replaceTileName should be...

/fill ~ ~ ~ ~1 ~1 ~1 redstone_block 0 replace command_block 0
No blocks filled.

/fill ~ ~ ~ ~1 ~1 ~1 command_block 0 replace redstone_block 0
Data tag parsing failed: Invalid tag encountered, expected '{' as first char.