Negative Mitzvah 106
We are forbidden to exchange one animal that has been designated for a sacrifice with another animal
Leviticus 27:10 "He shall not exchange it or transfer it"
We save our nicest clothes to wear on Shabbat, holidays or special occasions. We will usually not wear these clothes to school or while playing. The same applies to sacrifices.
If a person has set aside an animal to be offered as a sacrifice, that animal is considered special. Once it has been designated as a sacrifice, it can only be used for that purpose. We are not allowed to exchange or substitute another animal for one that has been set aside to be sacrificed.
Positive Mitzvah 87
Sanctity of a Substituted Animal designated for an Offering
Leviticus 27:10 "Then both it (the original animal) and its substitute shall be holy"
An animal designated for sacrifice is considered special. If that animal has been exchanged for another, the Torah commands us to consider both of them as holy (see Negative Mitzvah 106).