The change of sponsor doesn't surprise me as money does talk, but the fact it's Slazenger does! They seemed to be getting rid of their sponsored players recently, so is a resurgence on the cards? With their heritage it would be a shame to see Slazenger go, but they have gone off the boil in recently years (not helped by the sports direct association).
As for Adidas I think they have Willey on their books still (along with Broad and Southee) so will still have a presence on the international stage, without a real big name player giving them regular air time.