Thanks for this update. The is one think I would like to comment. Why do you limit developers to use only one datastore per app? It mite be much better to include additional parameter in datastore API to allow developers to access data in both datastores. As application developer it's hard to tell before hand what replication method better for any specific application. Personally I prefer more reliable solution for any project. But in some large applications there mite be some critical data and some less important data.
Speaking about current implementation. It's really weird that we do not have any way to migrate to High Replication Datastore without creating new application. It mite look not a big issue from Google's point of view, but that makes a lot of troubles to those developers, who use appspot domain to host applications. New application name means new domain name, etc.