From skyteam website:
"What will happen to my SkyTeam benefits, like earning or using frequent flyer miles, airport lounges, SkyPriority or Go Around the World pass? And will that change immediately after 1 January 2019?
Customers are our priority. Things will change, but it will not happen overnight. SkyTeam and China Southern have agreed to work together throughout 2019 to ensure a seamless transition for all of our customers. We will keep you updated in the coming weeks and months."
While a bit hard to read between the lines I think it is likely skyteam members will still be able to get lounge access and earn mileage for a while longer. I also doubt skyteam wont announce at such short notice end of benefitsbPlus China Southern still gains from giving benefits to skyteam elite plus and their own members continuing to use the skyteam network for the short to medium term. What China southern wants is to develop more with AA and other non skyteam members where skyteam agreements were limiting it currently from doing that.
I think Oneworld membership is far from certain considering CZ has 2 hubs So close to Hong Kong CX(CAN and SZX...SZX CZ is growing its international network). CX could easily veto them joining. So if that happens CZ could more likely be like Emirates which selectively allows partnerships.
At least I feel more confident now booking next month for Chinese New Years domestic flight to and from Chongqing, getting lounge access as well as other benefits.