I had plans to modify my second DW056 impact driver and turn it into a DW057 1/2" impact wrench a few months ago. After researching a bit more I decided against it.
1. I priced out the parts for the mod (housing and anvil). With shipping and taxes it wasn't that much more to buy a brand new DC820B which was on sale for $109 CAN or DW059B which was on sale for $129 CAN and both were in stock.
2. By doing the mod, although I knew it would work, it wasn't guaranteed. Plus there was no guarantee that it would do what I wanted it to do as in number 3.
3. Torque. DW056 - 98 in-lbs. DC820B - 145 in-lbs. DW059 - 300 in-lbs. I needed a tool that could easily drive 1/2" x 6" lag screws into pressure treated 6x6's or remove lug nuts effortlessly.
I ended up buying a DW059 to complement my two DW056's. I'm happy that I did because I have used both impact drivers at the same time (less frequent bit changes).