The part that the 6 screws thread into? You removed any and all snap rings that are in place to retain it, right? If so, I've had one get stuck in the hub before too. I threaded in 2 of the screws at opposite positions (lets say 3 and 9) and used them as a handle to wiggle it out. It wasn't moving at first so I used some small vice grips and clamped them to a screw, on the shank of the screw but right against the screw head. Then used a small hammer to tap on the vice grips effectively moving the stuck hub part (I can't think of its proper name) enough where I could just remove it by hand. That's one way to try removing it. No hard hammering or you could risk breaking the screw off.
Otherwise, leaving the old one in, as long as it mates properly with the new hub could be fine for years. Eventually you'll have to remove it for bearing maintenance or brake replacement though.