If mass-energy equivalence is taken into account when water is cooled to form ice, then:
Argument 1. The mass of the water should remain unchanged as the energy lost during the phase change is accounted by lowering of potential energy of the molecules of water.
Argument 2. When water is cooled to form ice, its thermal energy decreases. By mass-energy equivalence, mass should decrease.
Which one do you think is correct?