If you want to replace a WHS data disk, just add the new disk (using the WHS Management Console) and then remove the old disk (again using the WHS Management Console). Don't physically remove the old disk until WHS says it's safe to do so.
Replacing the WHS System disk (on a planned basis) is a bit more complicated. There's a couple ways to do it, but each has its advantages/disadvantages. As with any file server, it's best to have backups. That gives you the most options.