The Court said the proposed change was effectively retrospective because (as summarised by the supreme court ruling):

The Court of Appeal upheld the Administrative Court’s judgment that it is not within the power conferred on the Secretary of State by the Energy Act 2008 to reduce the tariff paid for electricity generated by small-scale solar photovoltaic generators, in respect of installations becoming eligible for payment prior to the coming into force of the modification.

This gives comfort that the government cannot change tariff levels applicable to systems installed before the new tariff levels have been approved by parliament.