At the end of Episode 3, Yoda tells Obi Wan that he has some special training for him, that he'll teach him how to commune with Qui Gon. Setting things up for how Obi Wan was able to return as a Force Ghost. We also saw Yoda teaching an entire group of Younglings at the Jedi temple, so it's possible Obi Wan also had some training under him, prior to becoming a Padawan to Qui Gon. Mark Hamill's accident happened in January 1977, while the first Star Wars movie came out in May that same year. So yeah, when they returned to film Empire, the Wampa scene was likely written as a way play up where the scars came from.