DAX was joined with the Jadzia for six years before she died. Surely a rich life, like that of Torias (1 1/2 years), but worthy and memorable nonetheless.
Ok, so, clearly you’ve never seen or heard of Star Trek before (if I’m wrong, I apologize), or you’d know and understand all of this, so I’ll explain:
Trill society has a subset who can be joined with Trill symbiots. These work/slug-like creatures can live for centuries, even longer. They “join” with the humanoid Trill and become a unified “person”, different and unique from either the host or symbiot which came before the Union.
So, once Dax (the symbiot) seperated from Curzon when he died and joined with Jadzia, was that a “new person”? Well… that’s debatable. Jadzia, the new host, was certainly changed, and Dax, the symbiot, certainly had some ”newness” to contend with. But… Jadzia was still Jadzia… plus Dax. It’s like having a new person, deeply nestled inside of your mind and soul. Does that, indeed, constitute a “new person”? Well… it would certainly demarcate many new and notable differences… but also much that remains the same.
It complicated. It’s not black-and-white, and it (probably) depends on individual experiences rather that a general rule.