So when a jinchuriki completely tames a bijuu, (Like Bee), does the bijuu stay that way for the rest of its existence, or does it revert back to its wild nature if it is removed from the perfect host?
What about if it is removed, and then replaced? Would Bee have to go under the same training again to train it?
well, with the bee thing, there's a whole other dynamic other than "completely taming". Bee and the hachibi trust each other, and have a friendship. Naruto and the Kyuubi don't have a trust, friendship, or any other bond other than hatred and the seal. so if the Hachibi was removed from Bee and placed into someone else, the Hachibi would most likely revert to how it was. if it was taken out, and then put back into Bee (makes no sense, but whatever), Bee would probably still have the control that he has due to their closeness.
if the Kyuubi was taken out of Naruto and put into someone else, it would try to kill them. if the Kyuubi was taken out of Naruto and put back in (again, makes no sense), I'm not actually sure how that would work. Since naruto gained a new seal after defeating it, Naruto may have to go through that procedure again.