AJ,
The short answer is yes. You can change the 4 and 6 inch barrels out. The trick is that you really need to start with a new barrel that hasn't been previously mounted. That makes things a lot easier.
You could do it yourself, but there are tricks to prevent warping the frame when unscrewing the old barrel. Additionally, the new barrel is unlikely to screw in and align correctly on the first try. Filing or milling the barrel shoulder that meets the frame a little at a time until a snug fit is achieve is a matter of patience.
Once the barrel is mounted, it's time to adjust the flash gap by properly milling the forcing cone. Good measurements and math can make this a single pass job. A final check that everything is square is done before driving the barrel pin into place.
If a gunsmith charges you $150 for it, that would be about right to cover his labor and wear on the tools.