Hi, and thanks for the questions!
Yes, now is the time to start experimenting. Actually, the new Harbour rules are already in place. So you can already submit apps with sandboxing profiles to Harbour, but the profiles won’t have any effect yet. And the organization name does not have to start with harbour 
For those who are better at reading code than English, here’s the commit, defining the new rules: https://github.com/sailfishos/sdk-harbour-rpmvalidator/commit/9988aa5a06ea3d30193e7548a12016d004d4e353
As for migration - yes, we definitely should write some kind of guide. For now, I think the best thing to do is to just move the configuration files from the old directory to the new. It’s easy to do as your applications are not yet sandboxed.