Yes big dam do redistribute mass and alter the Earth's rotation. But by my rough calculations the Three Gorges Dam should lengthen the day by roughly 0.05 to 0.06 microseconds. So around 1 second every 50,000 years. You are more likely to get late to work because your alarm run out of battery :-)
These dams have drawbacks, but the planets speed is so far down the list. The rivers ecosystem and the watershed would be much more impacted. Not that they can’t take those into account and try to minimize, but it is an inevitable side effect