In reality you give the same programmer an update to the existing spec, and they change the code to implement the difference. Which is exactly what the thing in OP is doing, and exactly what should be done. There's simply no reason to regenerate the result.

The entire thing about determinism is a red herring, because 1) it's not determinism but prompt instability, and 2) prompt instability doesn't matter because of the above. Intelligence (both human and machine) is not a formal domain, your inputs lack formal syntax, and that's fine. For some reason this basic concept creates endless confusion everywhere.