It is forbidden to substitute another animal for a sacrificial animal, and if one does so both are holy, as it says "[And if it is an animal of which sacrifices can be given to Ha-Shem, any [such] that he gives to Ha-Shem shall be holy;] he shall not exchange it and he shall not substitute for it... and if he substitutes animal for animal it and its substitute shall be holy".¹ It is forbidden to change any sacred thing to a sacred thing of another type, as it says "But a firstborn... no man shall sanctify it".^2,a

Sources:
1. Lev. 27:9-10	a. 1:1; 4:11
2. Lev. 27:26