Indeed, both AD and CS pcb files use the
Microsoft Compound Document Format, which can be opened in 7zip.
Thanks for the link to the (incomplete) cross-reference of AD stream names and the CS hashed stream names.
Perhaps stream names are the only change blocking two-way compatibility between AD and CS.
Or perhaps there is further obfuscation.
Have you tested that? Do you have a search-replace script to share?
When converting a cspcbdoc to pcbdoc in AD, there are definitely more substantial file differences. But that might just be cruft.