This has been discussed before and I believe we have yet to see any reports that pax who upgraded to bc, have actually been denied entry to the TK Business class lounge. Of course TK would prefer you to keep away from the lounges, so I would expect this to be deliberate misleading info or by agents who have no knowledge or experience with lounge access. I bet if you turn up at the TK Business class lounge with your BP for a flight in bc - upgraded ticket or not - you will be allowed in, just like other pax with business class seat.