The Energy Labelling Regulation provides in Article 4(4) that: A product for which changes are made that are relevant for the label or the product information sheet shall be considered to be a new model. The supplier shall indicate in the database when it no longer places on the market units of a model.
Thus, if the cover leads to a significant change in the performance of the refrigerator, then the refrigerator with the cover should be considered as a new product. This is regardless of the fact that the cover is sold independently from the refrigerator and that it is installed at the premises of the customer. In that case, this new product (the refrigerator with the cover) should be registered in EPREL and a new label should be provided.
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.