Another question:
at which precise moment does a pig become porc (or a cow beef)?
When it gets slaughtered? When it gets divided into parts?

At the point where it passed from the hands of the Saxon peasant to become an item for the table of the Norman overlords.
On that principle, I'd expect when (it was/they were) made into the cut that was to be cooked. I suppose the cooks and servants in the Hall would be using the jargon of their masters when dealing with an item for "the boss".
My guess.

Good guess, Dave. It is confirmed somewhere in Scott's Ivanhoe where a Saxon complains about changing old alderman Ox's name to Beef when entering the Norman's table.

Sorry to enter this so late but I was abroad some days.