Same behavior as before except that the undo/redo actions don't record the non cubic replacing (good thing).
I did notice another issue though:
If you don't replace, but just do a normal fill with non cubic shapes. It doesn't fill (which is good) but it still acts as "progress" and moves on the to be filled blocks to the next step. When you after a few tries, use a cubic block, you'll have an area that doesn't get filled.
Undo and Redo actions don't record the fake filling with non cubic shapes though, so that's good.