Stances can't be replaced, and thus are usually supposed to remain useful (including by scaling), having a higher level stance that makes a previous one obsolete is therefore questionable. One concept would be to make the low level stance scale into the higher level one automatically (and gradually... I despise unnecessary granularity) and simply drop the higher level stances that are made redundant by this. Another would be to power up the higher level stances but make the lower level stances they replace as specific pre-requisites to them.
EDIT September, 2010 (for cross-referencing purposes): Another Shield based discipline, Steel Mountain.