Parmandur
Book-Friend, he/him
The playtest FAQ is quite clear, really:If only we could with what WotC says backwards compatibility means, specifically. Unfortunately, they didn't really provide a concise answer.
What does backward compatible mean?
It means that fifth edition adventures and supplements will work in One D&D. For example, if you want to run Curse of Strahd in One D&D, that book will work with the new versions of the core rulebooks. Our goal is for you to keep enjoying the content you already have and make it even better. You’ll see this in action through the playtest materials, which you will be able to provide feedback on.