Historical Rules · 10 June 2022
Rule 712.4b
Official source text from the 10 June 2022 Rules release
Normally, a transforming double-faced spell has its front face up while on the stack and has only the characteristics of its front face. However, if an effect allows a player to cast a transforming double-faced card “transformed,” the resulting spell will have its back face up and have only the characteristics of its back face. Its mana value is calculated using the mana cost of its front face. A copy of a transforming double-faced spell with its back face up has mana value 0.