If someone would guide you through recalling a memory of a pastlife that feels as real as the memories that you recall from your present life how much would that cause you to update?

Knowing how easily manipulable the human mind is, I wouldn't weight that as very strong evidence, especially when it comes to subjective feelings. As an example, humans modify their memories all the time without really realizing it, as in the case of people who point fingers at the wrong crime suspect and decades later are proven wrong.