Charles Ellicott Commentary Leviticus 27:10

"He shall not alter it, nor change it, a good for a bad, or a bad for a good: and if he shall at all change beast for beast, then both it and that for which it is changed shall be holy." — Leviticus 27:10 (ASV)

He shall not alter it, nor change it. —The identical animal vowed is to be delivered; no change whatever, even if it involves substituting a better animal for an inferior one, is permitted. The stress placed on this part of the enactment is indicated by the use here of two verbs with the same meaning. If the one who vows does change the animal he dedicated to the Lord, both animals, the one he originally vowed and the one he substituted for it, are equally holy and must be delivered to the sanctuary.