It is all very hard to find out with what we know of science right now. However, I find it hard to believe that going forward from the past would change your memories. In order for hyperspace to work, or for us to leave the universe, I think that we would have to hope that my theory that the most basic building blocks of matter exist independently of each other and are their own forms of a high dimension. If this is so, then leaving the universe would be possible, because we would in essence have our own universe. If we could leave the universe, then travelling through the fourth dimension might not be a problem, since you would be travelling independently of our universe. You could trael through time and reenter a prior version of the universe, change something, then travel back. That, of course, assumes that the fourth dimension does not have to contain a copy for every possible moment in time for every iteration of the universe, but instead keeps a memory of each point in time for the said universe. So, if you travel outside of the universe and back into it at a previous point in time, it could very well be like living a memory, with you being unable to change anything. Of course, under this model, you might not even be allowed to enter the memory under normal circumstances, simply because it would be a memory and not real.