Regarding the interfaces between engine, buildscripts and server: how about the masterserver publishes the currently available version as it did for CR, and the engine simply compares that version with the engine version, but the download URL the engine uses to fetch an update is based on the old version? That way, the download server could either serve an individual update package for every supported old engine, serve the same cumulative update package under multiple names, respond with a 410 gone for too old engines when we run out of space, or continue to serve old update packages to old engines, requiring multiple update cycles for them. And we wouldn't have to decide on what we want to do until we actually produce the update, nor write  complicated code in the masterserver or the engine.