Legally being placed under the same corporate entity was a while ago. As is normal for big acquisitions, actually merging the businesses is a much slower process (allegedly part of the reasons BA's first days of operating at Heathrow Terminal 5 were a mess is that it was the first time BEA and BOAC were actually working together).

My point is just, from what I've heard the switch to Line Yahoo reflects a real integration of the businesses, not just a change of brand.