Exchange and Redemption of the Sacrifice
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Exchange (Temurah) is when a person has an animal dedicated for a sacrifice, which is fit for offering, and he wishes to offer another animal in its place. Therefore, he declares about another animal, 'Let this animal be in place of that one.' In such a case, the Torah states (Leviticus 27:10): 'He shall not exchange it nor substitute it, good for bad or bad for good; and if he does substitute animal for animal, both it and its substitute shall be holy.'
This means: A. The one who exchanges has transgressed the prohibition 'he shall not substitute it.' B. The sacrificial animal remains in its sanctity, and the substitute animal becomes holy with the same sanctity.
Redemption (Pidyon) is when a person has dedicated an animal for a sacrifice, and it becomes blemished. The law here is that since it is impossible to offer a blemished animal, the sacrificial animal must be redeemed for a non-sacred animal. The sacrificial animal exits its sanctity and becomes non-sacred, and the non-sacred animal enters sanctity in its place.
Source
Leviticus 27