Okay, the old Kid Buu vs. new Kid Buu logic....
Old Kid Buu + South Kai = Buff Buu.
Buff Buu + Dai Kai = Fat Buu.
Many, many, many years later....
Fat Buu - Mr. Buu = Gray Buu.
Now, were this algebra, we might assume that Mr. Buu = Dai Kai and that Gray Buu = Kid Buu + South Kai. But, this isn't the case, or Dai Kai would have come out of the subtraction, instead of Fat Buu (i.e., he would have looked different, and had Dai Kai's memories and motivations, such as maybe a desire to speak to the new Supreme Kai about everything that happened). So, then, Mr. Buu =/= Dai Kai, Mr. Buu = the goodness of Buu and Gray Buu = the evilness of Buu + most of his strength.
So now, Gray Buu eats Mr. Buu and becomes (what did you guys call this guy? I'll call him Pre-Z Buu, since he was before the Z fighters were absorbed). I'll assume he did this because he wanted Mr. Buu's energy, and not his influence on his personality. Kinda like a dog eating its placenta (sorry about the visual), the dog doesn't get the placenta back in the form of a placenta, just in the form of energy. He wanted to recombine him differently, see? so, we have Pre-Z Buu, oh, wait, Super Buu, now I remember. So, we have Super Buu, who goes on to absorb the Z fighters, but since they are taken out without become as... digested... as the Kais, we'll ignore his further transformations until Mr. Buu is separated from him. That's when he returns to Kid Buu (interestingly enough, not Gray Buu). Still, what's left must, logically speaking, be a different Kid Buu than the original one, unless Mr. Buu was given back all the strength that was separated from him when he first separated from Gray Buu (translation: he was given back what part of Buu was formed from the digestion of the South Kai). So, either Old Kid Buu =/= New Kid Buu, or Old Mr. Buu =/= New Mr. Buu.