[Update: The Chiefs have since confirmed it.]
I'm a bit surprised by Greenwood's release given the Chiefs linebacker options at the moment. I didn't have him slotted for a big role in 2013 (outside of special teams) but I'm not sure why the Chiefs would release him now.
Tanney and Mattison aren't big surprises. Tanney's lasting impression in Kansas City will be his trick shot video, which we can all watch one last time:
That video has slightly less views than AP had in all of March. So, yeah.
The Chiefs will continue tweaking the bottom end of the roster but most of the 2013 Chiefs are already on the team.