@pesos - we will not be testing with any starting point of unsupported configurations, I'm sorry.
We are sorry the announcement took a long time to come out, it was a complex issue to work through from our side.
You will need to be prepared to thoroughly test your own configuration when the feature comes out and work through your own migration plan - your configuration is unique most likely.
As highlighted in this post, ABP's are not going to allow you to host Exchange in the sense most of us agree on as 'hosting'. Enterprise, or on-prem Exchange, is not the right product for that. If you really want to host Exchange and sell hosted Exchange mailboxes, then you need to use the /hosting version and build a platform with that as the base.