I'm don't see the point of this proposal.  The fields of dataclasses are already mutable, so they can be updated just like any other object (no need for a special mechanism):

a = InventoryItem(name='Widget', unit_price=37.25, quantity_on_hand=10)

a.quantity_on_hand -= 1           # Sold one
a.unit_price *= 0.90              # Ten percent off sale

Also, it's not hard to directly take data from one instance to another:

a.name = master_catalog.name
a.unit_price = master_catalog.unit_price