I'd like to point out that in my OP, I also explicitly stated that when we get to Mother Winter's echelon, the rules seem to be different. Apparently there are some bedrock cosmic forces that have next to no turnover rate. However, these cosmic forces certainly have their influence affected by Humanity's choices. Granny doesn't have her walking stick any more, and that affects her ability to move about and influence things.
OTOH, her walking stick probably represents an aspect of her authority and power, and thus while wielded with a mortal will, that portion of her cosmic force is in some ways more directly influencing events on the ground as you might say.
Which sounds a bit like a mortal taking up power and responsibility over a certain aspect of reality and being influenced by that power and responsibility to the point that they morph into say, one of the Sidhe.
Good ole Granny is still around, and she hasn't "changed" but her direct influence over reality has, and in this hypothetical situation, we have a new immortal that has responsibility for/authority over an aspect of reality.