Replacement for Shimano Nexus 7 outer casing holder
If you've got the CJNX40 or CJ8S40 bracket then you'll need the piece. So yeah, order it. 

Frankly, if you just swapped the cog I don't understand why the cassette joint is different. But I'm assuming more happened than just the cog. Whatever the case, if the casing slot/area/thing on this bracket is bigger than the previous casing slot/area/thing then that holder unit makes up the difference.