Sorry this is potentially very obvious, but I want to be absolutely sure it will work before I spend so much CSC on one. Does anyone know if the Miracle Potion II will work to change a pet to a Yaherra? I want to change a pre-existing pet to one rather than adopt a new one.

If you go to the Zapper (just click the Adopt a Yaherra link) you have the option to either create a new pet, or change and existing one into a Yaherra :D

Oh, okay! I must've misunderstood the text on the page there, thank you for clearing that up for me!!