Quote Originally Posted by KnottedYet
Couldn't you keep the cranks and front derailleur and shifter and just change out a ring (or two)? (and adjust things afterwards, of course)
To a certain extent, yes. On a standard double crankset (typically 53/39) the smallest chainring you can mount on it is 38. You can also swap out the large ring to a 50, for instance.
A compact crankset uses a smaller spider arm (110 BCD) to hold the rings and typically come with 50/34 or 36. You can mount larger rings on a compact crankarm if you wish to make that change. One normally sets up a bike with either the standard or compact double depending on their strength & riding style but that's another completely separate subject matter.
You can use your front shifter and derailleur with either setup. No need for a compact specific front derailleur....standard front derailleur works perfectly on a compact double.