The Last Jedi would have the least amount of changes, structurally. But any reference to the Republic being destroyed, or the First Order taking over the galaxy within weeks, is removed. A reason for why the Resistance fleet doesn’t just escape to Republic space may need to be created. Perhaps the Supremacy is generating a gravity well, or we could add an interdictor to the fleet. Perhaps this could be a place for more radical ideas for the B-plot. When Leia refers to her “allies”, maybe she could be more explicitly referring to the Republic. Maybe they get a transmission from the Republic on Crait specifically saying they cannot intervene.
It could also be an out of the pot into the frying pan situation. The doo doo hit the fan after Hosnian got blown up, even if the New Republic survives they’ve probably got problems of their own. It would be pretty easy to justify the Republic not showing up on the basis that they’re also fighting the First Order, all across the galaxy.