If the earth warms significantly, the polar ice caps will melt. Water will move from the poles, near the earth's rotation axis, and will spread out around the globe. In principle, this will change the length of the day. Why? Will the length of the day increase or decrease?

The length of day will decrease

Explanation:

Think about it, if the tidal waves are monitered by the moon then the best way to do so is at night. The earth warms up, then the polar ice caps melt. The ice caps melt then there is more water to maintain. The moon has an effect on the tidal waves so I think that the length of days will decrease.