Sefer HaChinuch ספר החינוך

To not change the consecrated from a sacrifice to a sacrifice: To not change the consecrated from a sacrifice to a sacrifice, such that we turn a peace-offering back into a guilt-offering, or a guilt-offering we turn back into a sin-offering — there is a negative commandment in this and that which is similar to it. And about this thing is it stated (Leviticus 27:26), “a man may not consecrate it” — meaning that he not make the firstborn a burnt-offering nor a peace-offering nor any other sacrifice. And the received tradition comes that it is not specifically with a firstborn that the verse is concerned — that the law is the same for all that is consecrated [for] the altar. As so did they say in Sifra, Bechukotai, Section 8:3, “I only have the firstborn. From where [do we know] about all the consecrated that we do not change it from [one] holiness to [another] holiness? [Hence] we learn to say, ‘with a beast, a man may not consecrate it.’” It hints that with every consecrated animal — whether the consecration of the consecrated for the altar or even for the upkeep of the [Temple] — we do not change it from its holiness, but it is [to be] left like it is; and “a man may not consecrate it” is stated about everything.

That which we wrote above in the commandment of exchange (Sefer HaChinukh 352) is from the roots of the commandment — that all this is from the awe of the holy.

From the law of the commandment is, for example, that which they, may their memory be blessed, said (Mishneh Torah, Laws of Substitution 4:11) that if he consecrated [a beast] to the upkeep of the sanctuary, he should not change it to the upkeep of the altar. And they also said (Temurah 25a) that we are not to be tricky with a consecrated animal, to consecrate its embryo [with] a different holiness, but it is rather in the [same] holiness as its mother — as offspring of consecrated animals are consecrated from the innards of their mother. This is not the case with the firstborn, as the firstborn is consecrated upon its exit, as Scripture makes it depend on opening the womb. And therefore it is possible to be tricky with a firstborn before it is born, to consecrate it with a different holiness. And about this is it stated, “which becomes a firstborn of [...], a man may not consecrate it” — from when it becomes a firstborn, you may not consecrate it, but you can consecrate it in the belly. And therefore one who said, “If the one that is first is a male, behold it is a burnt-offering,” must sacrifice it as a burnt-offering. But he may not make it into a sacrifice of a peace-offering, since he cannot remove it from its holiness. As the holiness of peace-offerings is below that of the firstborn; such that the firstborn is only eaten by priests, whereas the peace-offerings [are eaten] by every person. And the rest of its details are in the fifth chapter of Temurah.