If the mass of the Earth were to become half, the acceleration due to gravity and consequently the downward force due to the Earth would decrease. Devices that use springs to measure a person’s mass would fail to give the correct result. This is due to the fact that these work on the principle that when a spring is pushed or pulled by a force it compresses or stretches proportionate to the force applied. Due to the difference in force exerted by gravity, the spring’s length would no longer alter as it did earlier.
Balance scales on the other hand will give you your correct mass. This is because the change in acceleration due to gravity affects both your mass as well as the weight to the same extent in relation to the unit you are being measured against, so the units will remain the same. If 45 pieces were required earlier as a counterbalance for your weight, the same 45 pieces will be required now.