I too bought a bike at Harris and they let me swap the bars, stem, and front derailleur, and would have swapped the casette if I desired. Their attitude is they take the build kit that is shipped with the bike, but since the parts are new they can also sell them to other customers so they'll customize the bike as you like, usually for no extra charge assuming you are substituting parts that are comparably priced. In my case I didn't need the ultegra crankset since I had a custom one, so they credited me for that. Most good bikes shops work this way on a $2000 plus bike.