I don't think merging timelines *is* the same as changing history. See when I hear of "changing history" I think of someone, like the Flash, who has the ability to timetravel, going back within the same timeline, and messing with it, possibly creating all kinds of weird paradoxes and problems with events taking place as they should. That is changing the history within that reality. Merging two timelines doesn't present the same paradoxes, because the two timelines just settle into a new consistent one through a "natural" process, natural in that it obey natural law of the DCU, kind of like humans building particle accelerators or fusion bombs and similar gadgets, simulating processes that occur without humans only inside stars or way back at the beginning of the universe etc. Hal just has to choose the right timelines to get the right effects.