I swapped a standard double for a compact double. All that's needed is the new crankset, assuming you find one that uses the same interface (square taper, ISIS, Octalink, external, etc.). A standard front derailleur can usually be set up to work fine with a compact. Mine has been working for a few years now, and the worst was a dropped chain to let me know I needed to set the limit screw a little tighter.