At this moment I can’t commit to provide such a tool. But we will maintain our branches like Symfony does. That is, all 1.0.x upgrades will remain strictly back-compatible. And 1.1.x may introduce something that is not BC. We will see if an upgrade tool can simplify the upgrading process. And if so, we will definitely provide such a tool.
I'm dreamin of a framework that could be upgraded automatically by calling App->upgrade($version) somewhere. And it connects to the website, finds necessary files and updates them…
Framework is different from applications like CMS because for CMS the main thing about BC is the data structure stored in database.
In fact, if you use a CMS and install some extensions, you will find the so-called automatic upgrading doesn't work very well. That's similar to the situation that a framework upgrade faces.
Anyway, we will try to keep BC as much as possible. And should there be BC problem, we will try every effort to make upgrading as easy as possible.