This question isn’t just for fans who write fan fiction. It’s also for the writers who change the narrative of classic games.
Let’s talk about canonicity.
In fiction, canon is the material accepted as officially part of the story in an individual universe of that story. It is often contrasted with, or used as the basis for, works of fan fiction. The alternative terms mythology, timeline, and continuity are often used, with the former being especially to refer to a richly detailed fictional canon requiring a large degree of suspension of disbelief (e.g. an entire imaginary world and history), while the latter two typically refers to a single arc where all events are directly connected chronologically.
With a game like Final Fantasy X, X-2 and the radio drama are apart of the lore. We accepted Final Fantasy X-2 in time. But the radio drama! Many people still can’t believe the radio drama is real.
But for simplicity, let’s take Final Fantasy X-2. Are we justified in thinking that Yuna would become a gunner instead of a summoner?