The transformer's secondary current rating will remain unchanged. So as the secondary voltage is raised by some percentage, the KVA rating also rises by that same percentage. Same logic for a lower voltage setting. However...
...as the number of secondary turns is reduced when the voltage is lowered, the primary current will be higher for a given secondary current. This will increase the I2R losses in the primary. The manufacturer may derate the transformer somewhat more than the reduction in KVA estimate. Or not, depending on their design margins.
Standards call for full nominal MVA rating at all taps and temperature rise test is done feeding the iron loss plus maximum copper loss at any tap.So usually you need not derate for lower voltage tappings.There can be exceptions where tap range is extensive say -25 %.
