I completed this same kind of transformation on my trek madone 5.0. I went from triple ultegra to double ultegra.
Change the right shifter for a double. Change the crankset for a double. I kept the same bb. Change the FD for a small one. The last thing is I could've keep my long rear-derailleur, but for the fun of it, I changed it for a small rear-derailleur. I bought used parts on ebay, except for the new cables.
Now I'm really happy with the new setting a plus I've learned more about bike mechanics, since I've done all the modifications.