What DevL means is that the pin holes on a front sight base (and the corresponding grooves in the barrel) aren't machined in to some print spec, which is what would be necessary to ensure that they would interchange from one barrel to another. Instead, the FSB is slipped onto a barrel, and the holes for the pins are drilled into the FSB and barrel at the same time. The spacing of the holes, and the exact depth that they go into the barrel, will vary slightly from rifle to rifle. This isn't normally seen as a problem, as manufacturers and the military consider the barrel ASSEMBLY to be one part, and if something on it is damaged, the entire assembly is replaced.
Obviously, that was fine for most of the AR's history, as there weren't many options for FSB replacements anyway. But that obviously makes replacing a FSB more difficult, unless you stick to a clamp-on type FSB, which mostly limits you to shaved front sights.
Can your FSB be replaced? Sure, but unless you are very familiar with the process, it's generally something best left to a gunsmith with machining experience, who probably already has a jig set up for this purpose. And, consider that the costs of doing this would negate any savings in buying a cast FSB in the first place.
-Troy