I'm thinking that maybe the nuke going off will just transport them back to the present, and the rest of the final season will be Losties + Others vs. Jacob's rival in the present.
I mean, I could be wrong. Maybe they did change the timeline. But that doesn't fit with how TV shows, including Lost, normally handle such things. The nuke going off is one of those classic cliffhanger mysteries that usually ends with nothing of consequence changing. I think Juliet is probably dead, and the nuke may have somehow transported them back to the present, but other than that I don't think anything will change.
It could be that but that would be a bit anti-climatic. And, on other shows, there might be the tendency to play it safe. With Lost, I'm not so sure.
I think we can be confident of a couple of the things, not all of those characters are dead. And, Jacob probably isn't dead. Jacob not being dead would be significant. And, I think would require time alteration. Maybe not, but other methods of survival might be a bit cheesey. Oh, he didn't actually bleed to death . . .
I bet we'll see an alternative history. It will have been changed yet will be recognizable. How? Jacob has manipulated events in time before to produce an outcome. He's been busy since. He brought those characters back in time for a reason!