If we change the test voltage and current of a short circuit test lab, there are some interesting facts. For example, if we change the test voltag from 690V to 415V while the current from 10kA to 12kA, the short-circuit test capacity will be changed from 0.69*10*1.732≈11.95MVA to 0.415*12*1.732≈8.63MVA，which means the impact to the grid will be diminished.
However, the difference of voltage and current affect test equipment in different ways. Nominated voltage level is generally determined by the insulation materials and distance, while current-carrying capacity is determined by the materials and section area of the conductor.
Considering the frequent transient over voltage in electrical test labs, the insulation level of test equipment is usually higher than nominal voltage, thus no matter the nominal voltage is 415V or 690V, the equipment insulation is always designed and manufactured in above 1.5kV level or similar.
For the current-carrying capacity part, theoretically, the section area of test equipment conductor will be larger for 12kA than 10kA, thus the equipment price will be higher. However, the design of test equipment shall also contains enough margin, to avoid current impact caused by voltage change or load bank adjustment fault. So if you just do 12kA short circuit test seldom in very short time (no more than 100ms maybe), the 10kA system may work, but if you want to do 12kA short circuit test frequently, then it is best for you to update the entire test system.