1st gen bumpers will fit, but there will be a 1" gap between the top of the bumper and the bottom of headlights (might not even be noticeable) However, the advantage of getting a 2nd gen bumper is that it should (most do but not all) use all 6 mounting points (2 a cross the front and 2 on each side of the body mount.
If you get a 1st gen bumper, or one without the side supports, make you sure brace the front of the frame (4xinnovations makes a kit) , or when you hit something (or if you winch off it) the nuts will pull out of the frame. They are weld on, but they also give you 2 more mounting points.
If your talking about one that isnt a bumper, but just a brush guard, they should swap over easily. I think they share the front tow hooks mounting points.