Well, in Catholic theology, there are three steps or stages to demonic posession after typical temptation which happens to everyone.
Temptation or invitation-- ex. Regan playing with the ouija board possibly unwittilgly inviting demonic activity with her "imaginary friend" play.
1. Infestation--The scuttling in the attic is the first sign that demonic activity is escalating in the household . . . moreso than average.
2. Obsession--Similar to temptation but now more intense. Demons not only have the Ear of Regan as in temptation, but now her body becomes physically affected. This is where Regan starts to change. Perhaps this begins as early as when she is walkind down to the party and peeing (though I havent seen the film in a while so it may be slightly earlier or later.)
3. Full demonic possession--essentially everything that happens once Regan is bedridden and crazy.
As far as the demon in Africa, I would agree with Mike Farley that it was supposed to be the same demon. Essentially, by showing the demon before even establishing Regan and mom, the viewer can "read" Regan's medical problems for what they really are, demonic posession even while everyone around Regan merely thinks she is sick or crazy.