The purchasing contract has to be signed before that date. If it contains an option for delivery of transformers in the future with parameters not meeting the requirements of the regulation, these transformers are only exempt if the contract specifies their number and a clearly limited period in which this option is valid. An agreement between manufacturer and customer just indicating the intention to buy non-compliant transformers for indefinite time cannot be considered to create 'purchased' transformers that would be exempted.
Disclaimer: Please note that the European Commission cannot provide a legally binding interpretation of the EU legislation, as this is the sole competence of the European Court of Justice. Any remarks from the European Commission services are without prejudice to the position the Commission might take should related cases arise in a procedure before the Court of Justice.