Negative commandment · Lo Ta’aseh #107 of 613
Not to change the designation of a consecrated animal from one sacrifice to another
שלא לשנות את הקודשים מקרבן לקרבן
The verse · Vayikra 27:26
אַךְ־בְּכוֹר אֲשֶׁר־יְבֻכַּר לַה׳ בִּבְהֵמָה לֹא־יַקְדִּישׁ אִישׁ אֹתוֹ אִם־שׁוֹר אִם־שֶׂה לַה׳ הוּא
“Only the firstling of the beasts, which should be the Lord’s firstling, no man shall dedicate that; whether it be ox, or sheep: it is the Lord’s.”
The Rambam’s words
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.
שלא לשנות את הקודשים מקרבן לקרבן, שנאמר בבכור "לא יקדיש איש, אותו" (ויקרא כז,כו); כלומר, לא יקדישנו לקרבן אחר.
Why · Sefer HaChinuch #356
To not change the consecrated from a sacrifice to a sacrifice
“It is from the roots of the commandment that G-d, blessed be He, wanted to instill His fear in the hearts of people in all matters of sanctity, and as I wrote in the building of the holy House and its vessels in the Order of Vayikchu Li Terumah (Sefer HaChinukh 95).”
Who & when
“And this prohibition is practiced in every place and at all times by males and females.”
Where in the Mishneh Torah
Hilchot Gifts to the Poor הלכות מתנות עניים