As someone who has a bike with 5700 and one with 6700, I would keep your new bike as is and sell what you can salvage off the Cannondale on ebay. There is not much difference between the 105 and Ultegra other than weight and it's going to cost more than it's worth in my opinion to pay an LBS to do the work and have to buy a new matching shifter. I would rather have the shiny new uncrashed 105 than old crashed Ultegra and best to mentally move on completely from the accident. that's my take on it