Upgrading from OS X 10.10 Yosemite Developer Preview
These are questions that can't really be answered until after it happens. They are the risks that you take when doing what you describe. You can look at past examples of what Apple has done or find people who did the same as what you want to do now with previous versions, however none of that will be fact as we just don't know what Apple will do this time around until they do it.

As you mentioned, this is the Developer's preview. If you're not a developer then you shouldn't be installing it -- but you don't need me to tell you that. As for the public beta program, I think you misunderstood it, or else I did. I understood it as the first million did not get access to the developer preview version, but will get access to the public beta version when it is released later during the summer.

This would make sense as I'm sure they don't want to give a million essentially consumers access to the first developer release until after the developers have had some time to review and the first few batches of bugs get worked out. Once it's a little more stable and tested, then they will give out access to the public beta.

A few quotes from the beta site:

The beta program is limited to the first million, not the first million get the developer's preview:


  Join the OS X Beta Program and help make the next release of OS X our
  best yet. Install the latest pre-release software, try it out, and
  submit your feedback. The program is limited to the first one million
  people who sign up, so join today.


The beta will only become available at a later date, not now:


  OS X Yosemite will be available this fall. The OS X Beta Program lets
  you take part in shaping it by test-driving pre-release versions and
  letting us know what you think. Your comments will help us make OS X
  better for all Mac users. Sign up today, and the beta software will be
  available for you to install soon.