In a letter to the editor of The Economist, Moritz Kraemer, sovereign chief ratings officer of S&P Global Ratings, clarifies what it would mean for France to re-denominate French debt:

Buttonwood wondered whether Marine Le Pen’s plan to re-denominate French government euro bonds into new francs might constitute a sovereign default (January 14th). There is no ambiguity here: it would. If an issuer does not adhere to the contractual obligations to its creditors, including payment in the currency stipulated, S&P Global Ratings would declare a default. Our current AA rating on France suggests, however, that such a turn of events is highly unlikely.