Lots of crazy speculation recently in this thread. From the official posts and a little crazy speculation of my own, I think that the engine will run automatically based on what map is being used, so Black Sea would remain with all its missions and campaigns and use the existing engine.
Nevada would use EDGE.
I dont think we'd pay for the engine, but rather the maps. So, in effect, if someone released a free map using EDGE, we'd get to use EDGE for nothing.