Whatever a person is willing to substitute at the last moment was only ever a preference.
A vow is tested at delivery, never at the announcement, because announcing costs nothing and handing over the sound animal costs everything the flock is worth. The gap between the two is where a should lives, and the quieter the exchange, the more certain it is that the word never bound anyone.
The swap trains the man making it before it ever reaches the altar. He learns that his own word is a draft, and that lesson carries into every promise after this one, since standards do not stay local to the room they were lowered in. Before a great King whose name is awesome among the nations, what he loses is not livestock but the ability to trust anything he says to himself.