The Tweet didn't specify if the base game owners are able to upgrade to the complete edition. It only confirmed next gen upgrade is free.
To compare, what Skyrim did is that the PS4 SE owners can upgrade to PS5 SE for free, but to upgrade to the PS5 anniversary edition for the extra contents you will need to pay extra dollars for them.
Also, CDPR clarified this before, in their words DLC means small updates like extra quests, alternative costumes, etc. While expansions are the term for the two big chunk additions, HoS and Blood and Wine.
That said, it is possible they will let base game owners to upgrade to the complete edition given they're generally rather consumer friendly in that regard. But it is not as clear as some folks claimed in the thread as of now imo.