Yep, big bang rewrites are always scary. Doing something more incremental would have been safer. One way to "safely" do a big-bang change would have been to us

Yep, big bang rewrites are always scary. Doing something more incremental would have been safer. One way to "safely" do a big-bang change would have been to use both architectures in parallel and sync back and forth. That way, if the new architecture fails, you still have the old one. Of course, that is lots of effort, and restricts new features (which may be a good thing anyway).