Business Reward tickets are a strange thing, they don't seem to be like normal revenue tickets. I once used them to buy a (Y) ticket for Mrs t, who at that time had Gold status. This did not reflect on her boarding pass and she had a difficult time convincing the lounge staff in all 3 airports she went through to give her access. And you also don't get tier or Skywards miles. And the call centre (who are usually very good) cannot help with any BR queries.
But you CAN use BR points to upgrade revenue tickets and in one case I was offered a part BR points and part cash payment option to upgrade (Y to J). Also seems to be available for J (revenue) to F upgrades (but I did not have sufficient BR points and on that occasion was not offered a BR 'points plus cash' option.
It's all a but mysterious (at least to me).