I don’t foresee any major changes in RhinoCommon, it’s a .NET style SDK so unless Microsoft drastically changes the .NET rules(very unlikely, bordering on nah-ah) it’ll just see incremental updates.
There probably will be plenty of cases where parts of the SDK will be marked as obsolete and replaced by different classes/methods, but that’s typically not something that severely hinders learning curves.
GH2 will be completely different from GH1 on the SDK level.