We will communicate more when the plan here is final. I can't make promises here, but I would expect 4.11 to ship with 0.8 support and for that to continue working in some form for at least a month after the first long-term supported 1.X Rift SDK becomes available. You will probably have to rebuild to add 1.X SDK support at that point, but I would expect the changes to be non-invasive.
We will share more detail as soon as we can. There is a good chance you will have to patch your 0.8-based engine with code from our 1.X-based Github repo as soon as the public SDK comes out. I would recommend planning for a quick turn-around when that happens.