I think there is two ways to look at this Jiga Watts. The first one is where you can actually change the past so it effects the future. If you were presumed dead in the future, then you could go back to the time right after you left and nothing would have been changed. However if you leave again, it will just be the same as before.
The second one is where you can’t change the past, and you’ll only be creating a parallel universe. What used to be your future is now your present, and since what used to be your present is now your past, you can’t change the past, so you’ll still be presumed dead at when you left off. And so you can still go back in the past and live there as usual, you won’t be changing the future.
I think that’s how those theories go.