Crankset upgrade
 

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.

Assuming that it is an integrated axle/external bearing bottom bracket crankset, the answer is probably not. And you probably won't even need to swap the front derailler either, unless you want the slightly lighter weight double version, in which case you might as well also swap out the rear derailler.

If the crankset is not of the aforementioned type, you will need a different bottom bracket as well as triple cranksets use a slightly different chainline than doubles.

Now might be a good time for you to research crank length and q-factors if you are into that stuff.