They never modernized the FCS because it was already a reliable system. However, hitting a stationary dime 2 miles away isn't the same thing as hitting a maneuvering ship 20 miles (32.1 km) away. I'm sure the Iowa is fully capable of hitting targets at that range, but not with the reliability that people boast about/exaggerate.
With the Mk-8 Range Keeper you could input information like targets range, course, speed, wind speed, barometric pressure, temperature, temperature of the powder, sea state, your course, speed, it even corrected for the rotation of the earth. Once a firing solution was achieved it was continuously updated in real time. It made no difference if your target was maneuvering the computer would plot where the target WOULD BE when the shells landed. It didn't matter how much you maneuvered as well. Once the firing solution was set and maintained any US battleship equipped with it could bring very accurate fire no matter what the conditions are. The USS West Virginia demonstrated that capability at the battle of the Suriago Strait where she achieved a firing solution at 30k yards but waited to fire at 23. She scored a direct hit on her first salvo and continued to land accurate fire at a range where the Japanese could not return fire. It was like clubbing baby seals.