Great question.
I am speculating but it doesn't look like anything particularly helpful will be interchangeable:
Lower - No, different size, shape and mechanisms for trigger system, magazine and bolt release.
Upper - No, different size, shape and mechanisms for bolt carrier, gas tube, gas block, charging handle and sling mounts
Barrel - Maybe, possible barrel and compensator could be re-used
Stock - No, different back plate and stock
Accessories - Yes, sights and possibly side rails. Different bottom rail size.
So, unfortunately just from a high level data sheet comparison it does not look like there is much interchangeability. However, I do think it would be possible for CZ to offer an upgrade kit for the 805 of specialized parts that would address the most common requested features like non-reciprocating charging handle, bolt release and 3 position gas block. That would be awesome but I would not count on it.
Here again, just guessing at this point and hopefully as more information becomes available we can update this list.