I know that this change won't happen, as it's too far along in the process, but I want to throw it out there.
Upgraded Holy Sword increases the chance and power of critical attacks. But many undead are crit immune anyway, so it won't make much difference against one of their most common foes. I (and many of us) have just been thinking about how this would boost our DPS in relation to other classes, so we weren't thinking about the whole crit-immunity thing.
I would suggest that Holy Sword, instead of increasing crit range, should increase crit vulnerability (i.e., make it act as though the enemy had less fortification. Say... 25% less). The end result against many foes would be similar: more critical hits. But now, those critical hits could work against undead.