Hey gour - we'll keep the upgrade path in mind, definitely. There'll be an upgrade guide, and I've just been talking with Sam about a static code checker which you can run on your existing codebase and detect deprecated/removed APIs. That'll be something for a community project, most likely - but I think will help immensely with the upgrade process.
Thank you Ingo for encouragement. Let me tell you that in the meantime your book arrived here and is waiting on the shelf that I finish some post-migration tasks after moving from Linux to (Free)PC-BSD...then we'll start learning 2.4. ;)