There is a thread about this somewhere. Found it, i know the thread title mentions a 777, but it would also apply to your 787-8 to 787-9 change...
https://www.flyertalk.com/forum/brit...s-777-a-2.html
It is when a route which only operates up to business class has an equipment change to a plane that now includes a first class cabin. You will be able to select a seat in the First class cabin but you will only receive a business class service, meal etc. Note however if the club world cabin is not full then they may move you to a club world seat and leave the First cabin empty for the flight.
Bri