Mythic Wild Shape (Su)Choose one form you can take using wild shape. Using wild shape to take this form doesn’t count against your daily uses of wild shape. You can select this ability multiple times. Each time you select it, choose another form to not count against your daily uses of wild shape. You must have the wild shape class feature to select this ability.
You choose a form you are able to take at the level you gain the path ability. AFAIK, you cannot retrain mythic path abilities or mythic feats, you would have to take the path ability again to select another form (that you can take at that level).