He is not to replace it or exchange it, either good for bad, or bad for good. But if he does exchange one animal for another, then both it and the one for which it is exchanged will become holy.
But if he is poorer than your valuation, then he is to be set before the kohen and the kohen will set a value for him, in keeping with what the one who made the vow can afford.
“If anyone commits a faithless act and sins unwittingly against the holy things of Adonai, then he is to bring his trespass offering to Adonai: a ram without blemish from the flock, according to your value in silver shekels after the shekel of the Sanctuary, for a trespass offering.