i recently replaced the upper control arm as an assenbly in my '04 explorer. inadvertantly i had stripped the stud that goes through the control arm.
i removed the ball joint from the replacement arm and installed a replacement ball joint. i also had to tighten the nuts on the ends of the shafts which had loosened up.
taking the car for a ride, the ball joing "popped"out of the control arm.
i re-installed the factory control arm with a new MOOG ball joint and all is well.

im my experience, the ball joint can be pressed out of both the factory and replacement arms. and can be replaced accordingly. Just be wary of a certain chain store`s ball joints.
the MOOG joint i uinstalled is just a hair oversized and is greaseable, so i forsee a long life.