"This reform--we decree it silliness without a name”
Like a sports team, a family is better off with uniformity of nomenclature. The French have made things extremely complicated...
It is retroactive only for children younger than 13, and even then only through a formal petition. Siblings must have the same last name. A husband cannot take the last name of his wife. In the case of a dispute between mother and father over what name to give their baby, the father wins.

There are jokes about the way the double-barreled last name will be put into effect. To ensure that the world at large knows that the baby's lineage results from the new law and not from historic cases often involving aristocracy, two hyphens will now be required.
So the name of a baby called Martin -- Dupont becomes "Martin double-tiret Dupont" or "Martin double-dash Dupont." "Double-dash" will not be pronounced in ordinary speech, according to the law's instructions, only in spelling the name.

I kind of like Double-Dash. I would pronounce it. (My father did often refer to me as "Ellyn Mary, quite contrary.")

While on the subject of names...when Embot gets married she will sound a lot like my favorite character in the Masterpiece boardgame: Bitsy Rich Wong Dobrowski Keyes. No double-dash needed.