Several people have expressed here that Encore6 should open not only Encore5 format files, but also the most recent Encore4 files (typically E455).
If my memory serves, there have been two formats in the E4 lifespan, and the earliest one was not readable by E5 – old E4 files needed to be converted using the latest E4 levels. But I don’t remember details (perhaps some incompatibilities with E5).
So, I would not expect that E6 could go back to “early E4” format.

As of Finale, Encore 5 did not open the Finale format, and apparently Overture doesn’t either. I would be surprized if the E6 plans were to include that format, taking into account that MusicXML is a much better approach to import in one editor the work prepared in another one.