The changing of the character had a more political, than spitirual reasoning behind it. The Okinawan masters wished to disassociate themselves with their Chinese counterparts in order to be more openly accepted by the Japanese.
That said, the character for "empty" does have better ring to it if one wishes to instill the Japanese Zen buddist philosophy.
All streams flow to the sea because it is lower than they are. Humility gives it its power.