I knew I'd read it somewhere... from the 2015 HY earnings call:

"One of the other things we've done is we've evaluated our 787 order book and it's likely that we will change some of our 787-9s to 787-8s, and that's for a couple of reasons. We see a better opportunity with the 8 and the seating capacity. The 8 doesn't have a first class; the 9 will have a first class. The configuration that we have the 787-9 in is effectively the same number of seats as the 787-8. So we think, on balance, given the trends we've seen in the market that there's a case for more 787-8s and a reduction in the number of 787-9s."