Box Coillider changes to trigger depending on speed of the object.

I'm trying to make a VR game with sword fighting, here is a webm of my game. http://i.4cdn.org/v/1456840247086.webm

I want the sword to be a box collider when the player isn't swinging it so the box collider pushes other rigidbodys.

Then I want the sword depending on how fast the sword is traveling (so probably at the speed of a hard swing) the box Collier on the sword changes to a trigger box collider. (probably have a variable in the inspector for how fast your sword needs to swing)

How could I accomplish this with C#?