I just upgraded my 2004 Classic with 520Cs front and back. It was not hard to do at all, I used the original transducers. I have an external temp sensor mounted too. Cabelas has a good deal on two units, GPS antenna, transducer, NMEA 2000 backbone, and an ethernet cable to joint the units. With a few extras, I did this for about $1250.00. Someone might be interested, but I also added a radio while working on it. I used a power boost antenna meant for inside an auto windshield use. I put it under the gunnel forward of the rodbox taking power off one of the courtesy lights. It gives good reception, and I have no antenna to break off when casting or putting on the cover.