Origin Giratina isn’t exactly the best example here. It had its available movepool altered, which is a different scenario than changing the raw stats of a move.
That said, sure Niantic could change Psystrike before it’s released. We saw them do that with Doom Desire for Jirachi right before it became available to Chicago Go Fest attendees. They did similar last-minute changes to Blaze Kick before Torchic CD, and then there was the buff to Sky Attack during one of the reruns for Lugia. There’s some history of them making changes to individual moves.
Then again, maybe they won’t change a thing. Like Psystrike, Psycho Boost was in the code before Deoxys was released. People were hoping the move would get altered (since it’s downright awful in PvE), yet it was released exactly as it was in the code.
So there’s no way to know for sure what Psystrike will look like until we’re closer to it going live. Until then, however, we only have what’s currently known in the code to guide our speculations and theorycrafting.