I am thinking they already swapped it and sold the S7's.  They just didn't fully stock and fire up the S9's so they wouldn't explode the difficulty.  There appears to be a very deliberate and steady diff increase going on these days.  Someone fires up a ton of gear towards the end of each diff jump and then lays off mid cycle before doing it again to produce a jump each time.  This next one looks to be a good size jump but whoever is doing it seems to have more and more hashrate with each diff step, which makes me think it must be bitmain.  There are a couple other miner manufacturers but they all seem more limited in how much gear they can produce (apparently not enough to sell any anyway).