Hi MonkeyM
The tiny pores in the bio-max adds a lot of surface area for BB the colonize. In time these pores get filled up with cellular matter from the bodies of the BB. This reduces surface area and causes each bead to not hold as many BB thus losing efficiency. By replacing half at a time you are giving the BB new area to colonize from the BB on the old beads.
Hope this helps, OS.
I think manufacturer's advice on the replacing of any media is for one reason and one reason alone: to sell you more media. It is sort of like the old shampoo advice: Shampoo, rinse and repeat (it was to sell twice as much shampoo!). Other than things being completely gunked up (it takes years for that to happen if one is diligent with their regular "swishing" maintenance) or falling to pieces, I don't bother replacing any of it--floss, foams, or ceramics.