Layer problem associated with block

I ran into a problem that I do not undestand why:

1. there is a block definition BlockA, say, two circles on layerA;
2. Open a new drawing, insert BlockA, Thus, layerA is introduced to the
drawing;
3. Later, I decide to redo BlockA, still two circles, but on LayerB (LayerA
in the block file is purged, not existed anymore);
4. I reinsert BlockA to the drawing to cause the BlockA's definition
redefined;
5. Now I want to purge LayerA, since no BlockReference or Block Definition
refereces LayerA, I should be able to purge, but I cannot. I deleted the
inserted Block reference, still can purge LayerA. I use code to loop through
the block's entities in definition (tow circles) to examine their Layer
property, it is LayerB, not LayerA, as I expected.
6. It is only after I purge the block definition (remember it DOES NOT have
reference to LayerA in its definition), LayerA is freed up for purging.

It seems, if you redefine a block definition, a layer that previously exists
in the old block definition is still associated to the block's new
definition, but you will not be able to find it by examine each
Block.Item(i)'s Layer property, it just exists in the drawing somewhere.

Can someone explain this.

What I want to do is that I have 1200 drawings with a particular block. I
need to redefine the block on new layer(s). In my batch updating procedure,
I successfully updated the block (new layer(s) were brought in with the new
block definition), but I cannot purge the old layer(s) used in old block
definition, although no entities, no blocks have reference to these layers.
They can only be purged if I purge the new block. Any idea how can I purge
those layers?