Here is what I can see on the net:
Money Shot: "GM said Opel will continue to provide some vehicles for its Buick brand and for its Holden unit in Australia." This probably means that the Cascada will continue to be sold under the Buick Badge until it is time to re-engineer the platform or until GM decides that they are losing too much money on it.
Money Shot: "Certain GM vehicles sold worldwide will continue to use Opel engineering platforms,"
Summary: No one really knows but it should be business as usual for awhile. I expect parts and service to be available forever. Opel is not going away, it is just going to have a new owner.
Please note that at this point in time the above are all WAGs.