If ground water is present then the pool already has hydrostatic pressure on it. As soon as the pool water level is reduced to a point where the hydrostatic pressure underneath the pool exceeds the weight of the pool and water in it, the pool can lift.
If yours has already lifted then it is not a good idea to do more than a partial drain. If you do need to completely drain the pool then you will need to arrange to also have the ground water removed continuously until the pool is once again full.