Retrofitting means you take the old lighting system out and putting the new one in. If you don't have anything covering your tank, well, that's another story. In that case, you usually need to either buy or make a topper that's got a feeding slot and then either buy the lighting fixture that will go on top of it or make a enclosure and then fit it with what you want. I'd advise going to http://www.drsfostersmith.com
to get some ideas, don't buy at Petsmart or Petco, the choices are very limited and a more expensive.
One question though...do you have to put a cooling fan in with the compact fluorescents when you retrofit?