I got the stl files from http://www.thingiverse.com/thing:194586
Though there wasn't info on actually swapping them out. So I am going to do a walk through.
Step 1: Printing Parts
Using the Printrbot simple I was able to print all the parts from the stl files provided.
Step 2: Removing the Y Carriage
I cut the zip ties and removed the wooden part so I could mount the motor mount. I also cut the sandpaper and rubber off. I then removed the screws from the motor.
Step 3: Installing Motor Mount
Screw in the motor mount. Make sure the notched/angled part is on the right. It allows you to tighten the set screw on the the actual gear. I learned this the hard way and had to take it apart and re-do this step. The photo is actually wrong, I later flipped it 180deg. Once this mount is attached you put the wooded y-axis back on.
Step 4: Belt Holders
You install these to the front and back like I did in the photos. You can do this before or after you mount the motor mount.
Step 5: Installing the Bearings and Gear.
You first put the gear on and then add the bearings. Don't tighten the gear until they are all lined up nice and flush. Then put the plate in that holds the bearings in place. Tighten the set screw after.
Step 6: Installing the Belt.
First run it thought the back. Then the gear. Then through the front. You then double I up on itself and pull it so it gets nice and snug. Then tighten it in the back, cut, tie off and tighten.
Step 7: Final Thoughts.
You can tighten the belt with the set screws in the back. It runs much better and doesn't slip.
This was all done at the tech shop in SF and you can visit their website at TechShop.ws