Backward compatibility

Sorry if this is a repeat of other posts.

I currently use Wavelab 8.5 and my employer has finally upgraded the whole organisation from WL 6 to 8.5.

However, I would dearly like to move to and work in Pro 9, but given the investment in a large broadcaster just getting to Vs 8.5, I doubt they’ll take another leap.

How much of WL 9 is NOT backward compatible? Clearly new things like the Mastering and the new plugins won’t be, but if a project/montage is setup with a minimum of WL 9’s unique features, can projects move comfortably between WL 8.5 and 9?

9 to 8.5 is safe if you don’t use new 9 features, such as mid/side
8.5 to 9 is always safe.

Thank you Philippe, I’m most grateful.