Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 107
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 107, with English translation by Maimonides (Rambam).
שלא לשנות את הקודשים מקרבן לקרבן, שנאמר בבכור "לא יקדיש איש, אותו" (ויקרא כז,כו); כלומר, לא יקדישנו לקרבן אחר.
Not to change the designation of a consecrated animal from one sacrifice to another, as [Leviticus 27:26] states regarding a firstling animal: "A person should not consecrate it" - i.e., he should not consecrate it as another sacrifice.