Do you have to be a Sorceror before you can swap out the Fiendish Sorcery trait for Prehensile Tail or Soul Seer? Or is it that only Sorcerors can have Prehensile Tails?
No, you could be any class and give up the Fiendish Sorcery for the tail. It is a trait that does nothing for many classes and prime for alternate rules.
You pick your race before class so the ability to swap it out exists without class choice.