Yes, there is.

The one I prefer, which you've mostly laid out, is: new passwords are entered as bcrypt(pw) and then stored as "B-$result", old passwords are re-hashed as bcrypt(hash) = bcrypt(md5(pw)) and stored as "M-$result", then your auth function works as follows:

    def auth(user, pw):
      hash = get_hash(user)
      if hash starts with "B-":
        return hash == bcrypt(pw)
      else if hash starts with "M-":
        return hash == bcrypt(md5(pw))
      else:
        # remove this once you've rehashed your entire database
        return hash == md5(pw)

The naïve solution is to skip the "B-"/"M-"/"" annotation but if you do that you've introduced a situation where attackers can login to old passwords using md5 leaked from another source.