351. Swap Meet: The prohibition against exchanging consecrated animals
He shall neither exchange it nor substitute for it… (Leviticus 27:10)
If an animal has been consecrated for use as a sacrifice, it may not be exchanged for another animal. The reason for this is out of respect for the Temple and its service. We should have proper awe for things that have become sanctified. If we try to remove an animal from its consecrated status, it shows that we do not take such things seriously.
As with the previous mitzvah, we should not consecrate animals today. If we do, it's effective and we then have a problem to clean up. This mitzvah is the topic of the Talmudic tractate of Temurah, starting at the very beginning (i.e., page 2a). It is codified in the Mishneh Torah in the first chapter of Hilchos Temurah and is #106 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos.