So there is a bike I'm thinking about purchasing for my first road racing bike. I have already raced one season on a borrowed bike, but it's definitely time to own my own. It has full Shimano Tiagra components, but I would like to upgrade the crankset from the triple that's on there now to a Shimano Ultegra double, which I prefer (and hopefully drop a little weight in the process.) My newbie question is, would I have to switch out anything other than the front derailleur to do this? thanks.