Bio-pellets will or should not reduce the water change schedule.
You still have to do them to replenish the elements from the salt as they are consumed. Bio-pellets also can help with phosphate issues and help to keep them low.
Using bio-pellets does not just mean you can set it and forget it and leave the tank alone. lol Still plenty of maintenance to do.
But to specifically answer your first question, nope you do not remove any of the pellets, the way it works is bacteria eat the pellets and consume nitrates so if you mess with the pellets then you will lose the bacteria. It takes a few weeks to start working cause the bacteria is building up. The only time you mess witht the pellets is to replace them when the level gets low. One of the most important things when using them is to keep the same flow and not tinker with it at all.
Hope that helps a bit.